mwifiex: make addba request command clean

uninitilized variable, such as .add_req_result might be magic stack
value. Initialize the structure to make it clean.

Signed-off-by: Xinming Hu <huxm@marvell.com>
Signed-off-by: Cathy Luo <cluo@marvell.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>