Update mailbox writes to work with VF devices.

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