wl1251: reorder wl1251_cmd_join() arguments

It's more common to have beacon interval before dtim period. Also use
bool instead of u8.

Signed-off-by: Kalle Valo <kalle.valo@nokia.com>
Reviewed-by: Vidhya Govindan <vidhya.govindan@nokia.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>