mwifiex: add P2P interface

Due to firmware design, driver needs to add a default P2P interface
to implement find phase and action frame handshake.

Signed-off-by: Stone Piao <piaoyun@marvell.com>
Signed-off-by: Bing Zhao <bzhao@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>