mwifiex: advertise correct beamforming information for VHT

Currently MU/SU beamformer and MU beamformee features are
not supported.

Hence this patch modifies VHT capability information accordingly.
Number of sounding dimensions should be zero in this case.

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>