brcmsmac: Introduce AMPDU sessions for assembling AMPDUs

AMPDU session allows MPDUs to be temporarily queued until either a full
AMPDU has been collected or circumstances dictate that transmission
should start with a partial AMPDU. Packets are added to the session by
calling brcms_c_ampdu_add_frame(). brcms_c_ampdu_finalize() should be
called to fix up the tx headers in the first and last packet before
adding the packets to the DMA ring. brmcs_c_sendampdu() is converted to
using AMPDU sessions.

This patch has no real value on it's own, but is needed in preparation
for elimination of the tx packet queue from brcmsmac.

Signed-off-by: Seth Forshee <seth.forshee@canonical.com>
Reviewed-by: Pieter-Paul Giesberts <pieterpg@broadcom.com>
Reviewed-by: Arend van Spriel <arend@broadcom.com>
Tested-by: Daniel Wagner <wagi@monom.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>