Move CHANWIDTH_* definitions from ieee80211_defs.h to defs.h

Move CHANWIDTH_* definitions from ieee80211_defs.h to defs.h

Move most of CHANWIDTH_* definitions from ieee80211_defs.h to defs.h as
the definitions are getting used mostly for internal purpose only. Also
change prefix of the definitions to CONF_OPER_CHWIDTH_* and update in
all the files accordingly.

Leave the couple of VHT-specific exceptions to use the old defines (the
reason why they were originally added as VHT values), to avoid use of
clearly marked configuration values in information elements. In
addition, use the defines instead of magic values where appropriate.

Signed-off-by: Aleti Nageshwar Reddy <quic_anageshw@quicinc.com>