ice: Increase mailbox receive queue length to maximum

Currently the PF's mailbox receive queue is only 512 entries. This fine,
but considering that all VF's mailbox send queues funnel into the PF's
single mailbox receive queue, let's increase it to the maximum size. This
will help prevent any possible bottleneck/slowdown occurring from the PF's
mailbox receive queue being full.

Signed-off-by: Lukasz Czapnik <lukasz.czapnik@intel.com>
Signed-off-by: Brett Creeley <brett.creeley@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>