mwifiex: update rx packet descriptor structure to match FW

RX packet descriptor structure has recently changed in FW.
This patch updates rxpd accordingly.

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>