pfsync: check IFF_DRV_RUNNING in the correct field

This flag is stored in if_drv_flags, not if_flags.

Reviewed by: glebius
MFC after: 1 week
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D33241

(cherry picked from commit 7b02a551f78ed5c4bd55d3b33da55524cd2317a6)