sfc: Cleanup reset code

Move more code from efx_reset() into efx_reset_down() and efx_reset_up().

Stop propagating MAC/PHY setting failures from efx_reset_down() and
efx_reset_up() as these should not be fatal.

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