mailbox: check ->last_tx_done for NULL in case of timer-based polling
It is allowed by code to register mailbox controller that sets txdone_poll
flag to request timer-based polling with missed ->last_tx_done() method.
If such thing happens and since presence of last_tx_done() is not checked
it will fail in hrtimer callback function txdone_hrtimer() when first
message will be transmitted.
This patch adds check for this method and logging of error on
registration of mailbox controller if it requested timer-based polling.