sfxge: support PERMIT_SET_MAC_WHEN_FILTERS_INSTALLED flag

Use flag on vadapter alloc when reported as a supported capability.
Use the slow device reset only when the capability is missing.

Submitted by: Richard Houldsworth <rhouldsworth at solarflare.com>
Sponsored by: Solarflare Communications, Inc.
MFC after: 2 days
Differential Revision: https://reviews.freebsd.org/D4387