ath5k: don't reset mcast filter when configuring the mode

We should not zero out the multicast hash when configuring
the operating mode, since a zero value means all multicast
frames will get dropped.  Also, ath5k_mode_setup() gets
called after any reset, so the hash already set up in
configure_filter() is lost.

Signed-off-by: Bob Copeland <me@bobcopeland.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>