Bluetooth: Controller: Fix casting in radio_aa_set for BASE0

We have previously fixed an issue with shifting and
assigning the value without a cast, but coverity
is still complaining about aa[1] and aa[0] with:

"Casting narrower unsigned aa[1] to wider signed type
int effectively tests its lower bound."

Using the common function, sys_get_le24, should fix this issue.

Signed-off-by: Emil Gydesen <emil.gydesen@nordicsemi.no>