mwifiex: add get_antenna support for cfg80211

Since commit de3bb771 ("cfg80211: add more warnings for inconsistent
ops") the wireless core warns if a driver implements a cfg80211 callback
but doesn't implements the inverse operation.

The mwifiex driver defines a .set_antenna handler but not a .get_antenna
so this not only makes the core to print a warning when creating a new
wiphy but also the antenna isn't reported to user-space apps such as iw.

This patch queries the antenna to the firmware so is properly reported to
user-space. With this patch, the wireless core does not warn anymore and:

$ iw phy phy0 info | grep Antennas
        Available Antennas: TX 0x3 RX 0x3
        Configured Antennas: TX 0x3 RX 0x3

Signed-off-by: Shengzhen Li <szli@marvell.com>
Signed-off-by: Amitkumar Karwar <akarwar@marvell.com>
[javier: expand the commit message]
Signed-off-by: Javier Martinez Canillas <javier@osg.samsung.com>
Tested-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>