wl1251: check return from call to wl1251_acx_arp_ip_filter

Currently the less than zero error check on ret is incorrect
as it is checking a far earlier ret assignment rather than the
return from the call to wl1251_acx_arp_ip_filter. Fix this by
adding in the missing assginment.

Detected by CoverityScan, CID#1164835 ("Logically dead code")

Fixes: 204cc5c4 ("wl1251: implement hardware ARP filtering")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>