regulatory: simplify freq_reg_info_regd

The function itself has dual-purpose: it can
retrieve from a given regdomain or from the
globally installed one. Change it to have a
single purpose only: to look up from a given
regdomain. Pass the correct regdomain in the
freq_reg_info() function instead.

This also changes the locking rules for it,
no locking is required any more.

Acked-by: Luis R. Rodriguez <mcgrof@do-not-panic.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>