sfc: Don't count RX checksum errors during loopback self-test

The loopback self-test checks that IP packets with incorrect checksums
are not altered when sent on a queue with checksum generation off.
These should not contribute to RX error statistics.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>