sfc: Merge efx_mcdi_mac_check_fault() and efx_mcdi_get_mac_faults()

The latter is only called by the former, which is a very short
wrapper.  Further, gcc 4.5 may currently wrongly warn that the
'faults' variable may be used uninitialised.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>