mac80211: Cancel pending probereq poll on beacon RX

While the probe request poll is expected to work, it looks like it
does not always result in getting a response. The exact reason for
this is unclear, but anyway, if we do receive a Beacon frame from our
AP, there is no need to disconnect based on the probereq poll. This
seems to help keep the connection bit more stable in cases where
beacon loss is occurring semi-frequently.

Signed-off-by: Jouni Malinen <jouni.malinen@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>