cfg80211: remove switch from __set_regdom()

We have control over the REGDOM_SET_BY_* macros passed
so remove the switch.

This patch has no functional changes.

Signed-off-by: Luis R. Rodriguez <lrodriguez@atheros.com>
Acked-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>