sfxge(4): removing unneeded assert

Remove ASSERT() from sfxge_link_mode_to_phy_cap() which causes an
unnecessary panic, even though the callers have proper error handling.

Submitted by: Andy Moreton <amoreton at solarflare.com>
Sponsored by: Solarflare Communications, Inc.
Differential Revision: https://reviews.freebsd.org/D18167