Currently we only translate error codes in efx_mcdi_poll(), but we
also need to do so in efx_mcdi_ev_cpl().

The reason we didn't notice before is that the MC firmware error codes
are mostly taken from Unix/Linux and no translation is necessary on
most architectures.  Make sure we notice any future failure by
changing the sign of resprc (matching the kernel convention) and BUG
if it's ever positive at command completion.
Signed-off-by: NBen Hutchings <bhutchings@solarflare.com>