Update register hot key logic, return EFI_ALREADY_START status if same hot key alread...

Update register hot key logic, return EFI_ALREADY_START status if same hot key already existed.

In current case, if one key was requested to register twice, browser will override old hot key with new one. This behavior is not user friendly.
Now update the logic, return EFI_ALREADY_STARTED for this case. If user still want to override it, he must unregistered it first.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Eric Dong <eric.dong@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@18604 6f19259b-4bc3-4df7-8a09-765794883524