fix(handoff): correct 8-bit modulo csum calculation

Fix the handoff 8-bit modulo checksum calculation to ensure we never get
a checksum larger than 8 bits. The previous calculation failed to
truncate the sum at the final step in update_checksum

Change-Id: Ice0b72eb139af90f416adeff157d337646d6201a
Signed-off-by: Harrison Mutai <harrison.mutai@arm.com>
3 files changed