wl1251: fix mixed up args for join

The join arguments are mixed up, passing beacon_interval instead of
channel and channel instead of beacon_interval. Fix them.

Signed-off-by: Grazvydas Ignotas <notasas@gmail.com>
Tested-by: Pali Rohár <pali.rohar@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>