sfc: Avoid mangling error codes in efx_test_loopback()

efx_test_loopback() used "|" to combine the results of the RX and TX
phases.  If both phases fail with different error codes, this results
in a bogus error code.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>