ps3-flash-util: Fix --db-write-half option

The ps3-flash-util --db-write-half option should use
os_area_db_set_16(), to write a 16 bit value.  This
bug cause ps3-flash-util to use os_area_db_set_32(),
so the value was put in a 32 bit location.

Signed-off-by: Geoff Levand <geoffrey.levand@am.sony.com>