sfxge(4): check NVRAM locking by partition ID

Tracking which partition is locked avoids being overly conservative
when EFX_NVRAM_xxx maps to more than one partition (depnding on the
current port number).

Submitted by: Andy Moreton <amoreton at solarflare.com>
Sponsored by: Solarflare Communications, Inc.
Differential Revision: https://reviews.freebsd.org/D18090