Bluetooth: Controller: Fix handling of odd SDU intervals in CIS central

Make sure iso_interval_us is in a multiple of 1.25 ms instead of directly
assigning it the value of SDU interval

Fixes EBQ failures in LL/CIS/CEN/BV-36-C and LL/CIS/CEN/BV-37-C

Signed-off-by: Troels Nilsson <trnn@demant.com>