mwifiex: fix setting of multicast filter

A small bug in this code was causing the ALLMULTI filter to be set
when in fact we were just wanting to program a selective multicast list
to the hardware.

Fix that bug and remove a redundant if condition in the code that
follows.

This fixes wakeup behaviour when multicast WOL is enabled. Previously,
all multicast packets would wake up the system. Now, only those that the
host intended to receive trigger wakeups.

Signed-off-by: Daniel Drake <dsd@laptop.org>
Cc: <stable@vger.kernel.org>
Acked-by: Bing Zhao <bzhao@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>