mwl8k: add internal API for managing AMPDU streams

In particular, we can now add, start, lookup, and remove streams.

Based on work by Nishant Sarmukadam <nishants@marvell.com> and
Pradeep Nemavat <pnemavat@marvell.com>.

Signed-off-by: Pradeep Nemavat <pnemavat@marvell.com>
Signed-off-by: Nishant Sarmukadam <nishants@marvell.com>
Signed-off-by: Brian Cavagnolo <brian@cozybit.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>