Bluetooth: Controller: Fix single switch timer minimum compare value

Fix implementation of single switch timer minimum compare
value requirement. Do not always add the maximum possible
radio latency duration, but check if the required compare
value is smaller, then use the remainder required as the
extra compare value. This fix reduces latencies to setup
radio receptions and fixes an assertion in lll_scan_aux
when radio_tmr_start_us() is checked for latencies.

Relates to commit bcd28e0a ("Bluetooth: Controller:
Fix sw switch single timer for spurious TXEN/RXEN").

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