chore(app/device_assignment): fix integer overflow reported by Coverity

Fix potential integer overflow error reported by Coverity.
This patch eliminates the problematic typecast from

   int -> unsigned long -> int

Signed-off-by: Mate Toth-Pal <mate.toth-pal@arm.com>
Change-Id: I6e7e7b1590c071c44efa19379ad0dbc6a84ee797
1 file changed