sfxge(4): fix comparison always true warning

Loopback type used as bit index has efx_loopback_type_t type
which is enum. clang complains that it is always true when it
is compared with qword (64 bit) bits number boundary.

Sponsored by: Solarflare Communications, Inc.
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D18228