mac80211: Allow work items to use existing channel type.

Narrow channel types can function within larger channel types.
So, use existing channel type for work items when possible.
This decreases hardware channel changes significantly when
using non NO_HT channel types on the operating channel.

Signed-off-by: Ben Greear <greearb@candelatech.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>