sfxge: improve error handling in ef10_ev_rx()

Ensure that checksum flags and L3/L4 fields are ignored
if lower level errors are reported in the event.

Remove checks for CRC0_ERR (bad iSCSI header CRC) and
CRC1_ERR (bad iSCSI payload or FCoE/FCoIP CRC) as they
are not used by any existing code.

Submitted by: Andy Moreton <amoreton at solarflare.com>
Sponsored by: Solarflare Communications, Inc.
MFC after: 2 days
Differential Revision: https://reviews.freebsd.org/D4975