net: iflib: sync isc_capenable to if_capenable

On SIOCSIFCAP, some bits in ifp->if_capenable may be toggled.
When this happens, apply the same change to isc_capenable, which
is the iflib private copy of if_capenable (for a subset of the
IFCAP_* bits). In this way the iflib drivers can check the bits
using isc_capenable rather than if_capenable. This is convenient
because the latter access requires an additional indirection
through the ifp, and it is also less likely to be in cache.

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