Bluetooth: Controller: Add 'U' to unsigned variable assignments

Add 'U' to a value when assigning it to an unsigned
variable.
MISRA-C rule 7.2

Relates to commit b97db52d ("misra-c: Add 'U' to
unsigned variable assignments in subsys/").

Signed-off-by: Vinayak Kariappa Chettimada <vich@nordicsemi.no>