fm: synchronize dequeue_fm_tx_cmd function

In dequeue_fm_tx_cmd function, there is race condition happened between
command credits decrement and event received for command sent. After
sending command to SoC, event received immediately and processed before
it decrement the command credits, hence command credits become zero and
it stuck in condition wait for command credits and FM failure happens.

Change-Id: Ibb2cf98ade08bbc9b25946d2368b1b7e428397bc