mwifiex: pass priv pointer instead of adapter

Pass mwifiex_private pointer directly to wmm_add_buf_txqueue()
instead of passing adapter and then deriving priv again in
mwifiex_get_priv_by_id(). This reduces a 'for' loop in TX path.

Signed-off-by: Avinash Patil <patila@marvell.com>
Signed-off-by: Bing Zhao <bzhao@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>