Bluetooth: Controller: Fix incorrect aux offset using single timer

Fix incorrect aux offset value in the aux pointer in the
primary advertising channel PDUs when using single timer
tIFS switching feature in nRF54Lx.

End time was not captured when only Tx follow by Tx happens
in the primary advertising channels. End time needs to be
always captured when using single timer so that elapsed
packet timer value is correctly accumulated, for use to
setup subsequent radio transmission or reception inside the
same radio event.

Relates to commit 373dc0db ("Bluetooth: Controller:
Single timer use Extended Advertising nRF54L support").

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