LinuxKPI: 802.11: fill regulatory_hint() with some life

Start implementing regulatory_hint() using a .c file based allocation
helper function so we could change structures in the future with
better chances to keep compatibility.
This sets wiphy->regd needed by various LinuxKPI based WiFi drivers.

Sponsored by: The FreeBSD Foundation

(cherry picked from commit 673d62fc359b0cb7a70af42c36d2fa54fb29452a)