wl1251: fix filtering support

This driver has a hack in cmd.c which effectively disables all filtering.
This seems to be triggering a firmware bug where it stops reporting any
rx packets after random time on some routers, which is eliminated (or at
least appears much more rarely) when filtering is on.
I have found that only BSSID filter needs to be disabled for association
to work, so disable only that instead of all filtering.

Signed-off-by: Grazvydas Ignotas <notasas@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>