Update mailbox writes to work with VF devices.

• Use alternate register locations for the data and control registers for VFs.
• Do a dummy read to force the writes to the mailbox data registers to post before the write to the control register on VFs.
• Do not check the PCI-e firmware register for errors on VFs.

Reviewed by: np
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D7483