cfg80211: export regulatory_hint_user() API

This is to help the hardware configured in world
roaming mode to save power when not connected to
any AP.

CRs-Fixed: 542802
Change-Id: Ia643d0e9848dcd486832973bd6dd186edd7bd4ea
Signed-off-by: Mihir Shete <smihir@codeaurora.org>
Signed-off-by: Will Huang <wilhuang@codeaurora.org>