mwifiex: support for event done interrupt

This patch adds support for writing CPU event interrupt done back
to device.
Patch also increases interrupt buffer ring size from 4 to 8.

Signed-off-by: Avinash Patil <patila@marvell.com>
Signed-off-by: Cathy Luo <cluo@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>