Bluetooth: Controller: Fix radio_tmr_start_us for single timer use

This commit addresses two bugs in use of single timer s/w
switch implementation, incorrect aux_offset in subsequent
ADV_EXT_IND and ISO receiver failing to receive second or
greater BIS subevents.

Fix radio_tmr_start_now implementation to consider the
initial latency in starting the event timer in single timer
use mode. This fixes the incorrect aux_offset in ADV_EXT_IND
PDUs.

Fix radio_tmr_start_us implementation for single timer use,
as the timer is reset on every radio end, and hence the
requested start_us has to be adjusted for the accumulated
last_end_pdu_us value. This fixes the BIS subevent
receptions.

Also, fix the maximum radio ISR latency value used in
radio_tmr_start_us to consider the maximum Rx chain delay
and maximum radio ramp up delays. 80 us + ~30 us + ~40 us
should be able to meet the 150 us tIFS duration.

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

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