e1000: fix interface capabilities management

The e1000 drivers (em, lem, igb) are currently looking at the
iflib copies of the capabilities bitvectors (scctx->isc_capabilities
and scctx->isc_capenable) rather than the ifnet ones
(ifp->if_capabilities and ifp->if_capenable). However, the latter
are the ones that are actually updated by ifconfig and that should
be used by the drivers during interface operation. The former are
set by the driver on interface attach (for iflib internal use)
and should not be used anymore by the driver.
This patch fixes the e1000 driver to use the correct bitvectors.

PR: 260068
Reviewed by: markj
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D33154