mwifiex: usb: transmit aggregation packets

Instead of using 4KB packet buffer for data transfer, new chipset have
more device memory. This patch try to aggregation packets in an 16KB
buffer. In this way, totally usb transaction cost will be reduced.

Thoughput test on usb 2.0 show both TCP TX and UPD TX promote ~40M,
from ~240M to ~280M.

This feature is default disabled, and can be enabled by module
parameter, like:
insmod mwifiex.ko aggr_ctrl=1

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