iwlwifi: clean up ht config a little

is_ht can be bool instead of u8, and there's
no need to use IWL_CHANNEL_WIDTH_* constants
in supported_chan_width when that could just
be named is_40mhz instead.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>