wl1251: don't build null data template in wl1251_op_config()

The bssid can be zero when null data template is set in wl1251_op_config().
It's enough, and especially safe, to set it once after association.

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