igb(4):
I210 restore functionality if pxeboot rom is enabled on this device.

r333345 attempted to determine if this code was needed or it was some kind
of work around for a problem. Turns out, its definitely a work around for
hardware locking and synchronization that manifests itself if the option
Rom is enabled and is selected as a boot device (there was a PXE attempt).

Reviewed by: mmacy
Differential Revision: https://reviews.freebsd.org/D15439