cfg80211: parse dfs region for internal regdb option

Add support for parsing and setting the dfs region (ETSI, FCC, JP)
when the internal regulatory database is used. Before this
the DFS region was being ignored even if present on the used
db.txt

Signed-off-by: Janusz Dziedzic <janusz.dziedzic@tieto.com>
Reviewed-by: Luis R. Rodriguez <mcgrof@do-not-panic.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>