cfg80211: move reg_is_world_roaming()

This will be used later by other code. This has no
functional change.

Tested-by: Ben Greear <greearb@candelatech.com>
Reported-by: Ben Greear <greearb@candelatech.com>
Reported-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: Luis R. Rodriguez <mcgrof@do-not-panic.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>