wl1251: wait for join command complete event

Poll for join command completion instead of waiting blindly for 10
msecs. There is a timeout of 100 msecs, if the command doesn't complete
by then, we return an error code.

Based on wl1271 patch 99d84c1d
by Luciano Coelho.

Signed-off-by: Grazvydas Ignotas <notasas@gmail.com>
Acked-by: Kalle Valo <kvalo@adurom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>