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).

drivers/SPI.cpp

@@ -83,6 +83,12 @@ void SPI::_do_construct()
         _peripheral->name = name;
     }
     core_util_critical_section_exit();
+
+#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
     // we don't need to _acquire at this stage.
     // this will be done anyway before any operation.
 }