Bluetooth: controller: Use proper transmitWindowDelay for slave

transmitWindowDelay is either 2.5ms or 3.75ms when AUX_CONNECT_REQ is
used (for 1M/2M and Coded respectively). It also uses the same unit
as transmitWindowOffset (1.25ms) so we can just simply add both.

Signed-off-by: Andrzej Kaczmarek <andrzej.kaczmarek@codecoup.pl>