Update mailbox writes to work with VF devices.

[jhb]

Summary

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.

Test Plan

• tested as part of the VF branch, also tested standalone on PF for basic testing (ping, netperf)