mwl8k: Remove redundant txq_idx

Ampdu stream index can be derived from stream->idx.
So we no longer need stream->txq_idx, hence removing it.

Signed-off-by: Nishant Sarmukadam <nishants@marvell.com>
Signed-off-by: Yogesh Ashok Powar <yogeshp@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>