SPI: Prime asynch transaction buffer on construction

SPI peripherals' asynch transaction buffers are now wrapped by
SingletonPtr, which needs to take the singleton_lock Mutex when first
accessed. If it was first accessed by an asynch transaction started from
IRQ, that would not be possible.

Add a SingletonPtr::get() call to the SPI construction process so that
the peripheral's buffer is fully constructed before any SPI methods can
be called, meaning asynch methods won't fail from IRQ.

(Other pre-existing synchronisation issues with async remain, but this
avoids a new trap in Mbed OS 5.12).

Author: kjbracey

drivers/SPI.cpp

@@ -81,6 +81,11 @@ void SPI::_do_construct()
     if (!_peripheral) {
         _peripheral = SPI::_alloc();
         _peripheral->name = name;
+#if DEVICE_SPI_ASYNCH && TRANSACTION_QUEUE_SIZE_SPI
+        // prime the SingletonPtr, so we don't have a problem trying to
+        // construct the buffer if asynch operation initiated from IRQ
+        _peripheral->transaction_buffer.get();
+#endif
     }
     core_util_critical_section_exit();
     // we don't need to _acquire at this stage.