ng_ether: Enter NET_EPOCH where required

We must enter NET_EPOCH before calling ether_output_frame(). Several of the
functions it calls (pfil_run_hooks, if_transmit) expect to be running in the
NET_EPOCH.

While here remove an unneeded EPOCH entry (which wasn't wide enough to cover
BRIDGE_INPUT).

PR: 248958
Reviewed by: glebius, bz (previous version), melifaro (previous version)
Tested by: manu
Differential Revision: https://reviews.freebsd.org/D26226