ice: Increase size of Mailbox receive queue for many VFs

ice: Increase size of Mailbox receive queue for many VFs

Currently we use the ICE_MBXQ_LEN for both the Mailbox send and receive
queues that are used to communicate with VFs. This is fine for the send
queue because the PF driver will lock the queue for every single send,
but for the Mailbox receive queue every VF is posting to its Mailbox
send queue and the hardware is then handing the message to the PF on its
Mailbox receive queue. This becomes a problem with many VFs because it
seems to overburden the Mailbox receive queue on the PF. Fix this by
increasing the Mailbox receive queue for the PF to 512 entries.

The number 512 was determined based on the number of VFs supported by
the device. We can have a total of 256 VFs so in the worst case this
allows the VFs to put 2 messages in the PFs Mailbox receive queue at the
same time.

Signed-off-by: Brett Creeley <brett.creeley@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>