power: fg-util: Fix the address mask for secure_write check

For the 16-bit FG peripheral register address check the lower-byte
for secure_access instead of the upper-byte.

CRs-Fixed: 2013922
Change-Id: I647aa4eadccb007e6e974eae65cef162c5a40a0d
Signed-off-by: Anirudh Ghayal <aghayal@codeaurora.org>