HT/VHT capabilities must be reported per each band supported by a radio,
not for all bands on a radio. Furthermore, driver better not assume
any capabilities and just use whetever is reported by device itself.

To support this, convert "get channels" command into "get band info"
command. Difference is that it may also carry HT/VHT capabilities along
with channels information.

While at it, also add "num_bitrates" field to "get band info" command,
for future use.
Signed-off-by: Igor Mitsyanko <igor.mitsyanko.os@quantenna.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>