net: iflib: let the drivers use isc_capenable

Since isc_capenable (private copy of ifp->if_capenable) is
now synchronized to if_capenable, use it in the drivers
when checking the IFCAP_* bits.
This results in better cache usage and avoids indirection
through the ifp pointer.

PR: 260068
Reviewed by: kbowling, gallatin
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D33156

(cherry picked from commit 52f45d8acee95199159b65a33c94142492c38e41)