mwifiex: add set_antenna handler support

This enables user to set mode of Tx/Rx path using "iw set antenna"
command. For non MIMO chips, the command will be used for selecting
specific antenna or configuring antenna diversity mode.

Signed-off-by: Amitkumar Karwar <akarwar@marvell.com>
Signed-off-by: Bing Zhao <bzhao@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>