companion_radio: tear down QSPI peripheral when its init fails (fixes #2827)

On nRF52 BLE companion builds, a failed external-flash init left BLE
completely dead: the node never advertised on the Elecrow ThinkNode M6
on any firmware version, even though the same unit runs fine over LoRa
and pairs over BLE under other firmware.

Root cause: when nrfx_qspi_init() fails (the M6 returns
NRFX_ERROR_TIMEOUT / 0xBAD0007 during QSPI activation), it has already
enabled the QSPI peripheral and enabled QSPI_IRQn in the NVIC at
priority 0 (reserved for the SoftDevice), and it does not clean any of
that up before returning. The very next step, the SoftDevice enable in
Bluefruit.begin(), then fails (returns false) and every later BLE call
reports NRF_ERROR_SOFTDEVICE_NOT_ENABLED, so the node never advertises.
USB companion builds are unaffected because they never enable the
SoftDevice.

Fix: on the QSPIFlash.begin() failure path, disable QSPI_IRQn and
power the QSPI peripheral back down before continuing. This only runs
on the failure path, so boards whose QSPI init succeeds are unaffected,
and BLE now comes up cleanly even when the external flash is absent or
fails to initialize.

Validated on a ThinkNode M6: with the QSPI init still timing out, the
SoftDevice now enables, the node advertises, and the MeshCore app pairs
and exchanges frames.

examples/companion_radio/main.cpp

@@ -139,6 +139,18 @@ void setup() {
     if (!QSPIFlash.begin()) {
       // debug output might not be available at this point, might be too early. maybe should fall back to InternalFS here?
       MESH_DEBUG_PRINTLN("CustomLFS_QSPIFlash: failed to initialize");
+    #if defined(NRF52_PLATFORM)
+      // A failed QSPI init (e.g. nrfx_qspi_init() returning NRFX_ERROR_TIMEOUT)
+      // leaves the QSPI peripheral enabled and QSPI_IRQn enabled in the NVIC
+      // (at priority 0, which is reserved for the SoftDevice) and does NOT clean
+      // up after itself. That poisons the subsequent SoftDevice enable
+      // (Bluefruit.begin()), so on BLE builds the node never advertises (#2827).
+      // Tear the peripheral down so the SoftDevice can start cleanly.
+      NVIC_DisableIRQ(QSPI_IRQn);
+      NVIC_ClearPendingIRQ(QSPI_IRQn);
+      NRF_QSPI->TASKS_DEACTIVATE = 1;
+      NRF_QSPI->ENABLE = 0;
+    #endif
     } else {
       MESH_DEBUG_PRINTLN("CustomLFS_QSPIFlash: initialized successfully");
     }