mwifiex: update bss band information

In recent commit "mwifiex: use cfg80211 dynamic scan..."
(7c6fa2a8..) scan table handling in driver is removed to
make use of cfg80211 dynamic scan table. Now driver sends
beacon buffers found in scanning directly to stack and parse
the buffer for requested BSS only during association.

Beacon buffer doesn't contain bss band information. Driver
gets it from firmware in separate tlv (chan_band_tlv).
Currently since we don't inform stack about bss bandinfo,
there is an issue with 5GHz association.

Use "priv" field of struct cfg80211_bss to store bandinfo.
This fixes 5GHz association issue.

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>