igb/igbvf: don't give up

The driver shouldn't just 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: Mitch Williams <mitch.a.williams@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>