The driver shouldn't give up if it fails to get the hardware mailbox lock.
This can happen in a situation where the PF-VF communication channel is
heavily loaded and causes complete communications failure between the PF
and VF drivers.
Add a counter and a delay. The driver will now retry ten times,
waiting one millisecond between retries.
Signed-off-by: Wenzhuo Lu <wenzhuo.lu@intel.com>