net/wireless: Check for number of sub bands

WiFi Drivers can advertize the number of bands dynamically.
A check has been added to see if the band information is
present and only then the values are extracted.
Without this check, a kernel crash shall occur as we would
extract the band information though the driver does not
advertize.

Change-Id: Ia726a692a3c2dcef2fcff0a59363053524c5f806
Signed-off-by: Sunil Dutt <Undekari&lt;duttus@codeaurora.org>