net/sfc: fix promiscuous and allmulticast toggles errors

Returned errors of ethdev callbacks are negative. Internal
sfc functions return positive errors, so convert them to
negative value.

Fixes: 9039c8125730 ("ethdev: change promiscuous callbacks to return status")
Cc: stable@dpdk.org
Signed-off-by: Igor Romanov <igor.romanov@oktetlabs.ru>
Signed-off-by: Andrew Rybchenko <arybchenko@solarflare.com>