libertas: rename packetfilter to mac_control

The CMD_MAC_CONTROL can be used for other things than just filtering
packets, e.g. to enable and disable WMM. This uses the same term mac_control
for the define, the function and the shadow value in struct lbs_private.

Signed-off-by: Holger Schurig <hs4233@mail.mn-solutions.de>
Acked-by: Dan Williams <dcbw@redhat.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>