Bluetooth: Controller: Fix radio_tmr_start_us to be soft realtime

Fix radio_tmr_start_us to be soft realtime, if the requested
start in microseconds is in the past, setup the next
earliest expiry. This is to prevent radio not being started
and not let the LLL hanging. When setup to receive auxiliary
PDU in LLL scheduling, is_aux_sched is set and if radio does
not start and the window is requested to be pre-empted, then
it is blocked from being preempted due to the is_aux_sched
flag remaining to be set. This stalls the scanning
infinitely until explicit scan disable.

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