nl80211/cfg80211: add 5 and 10 MHz defines and wiphy flag

Add defines for 5 and 10 MHz channel width and fix channel
handling functions accordingly.

Also check for and report the WIPHY_FLAG_SUPPORTS_5_10_MHZ
capability.

Signed-off-by: Simon Wunderlich <siwu@hrz.tu-chemnitz.de>
Signed-off-by: Mathias Kretschmer <mathias.kretschmer@fokus.fraunhofer.de>
[fix spelling in comment]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>