Bluetooth: controller: Fix incorrect master role scheduling

Revert incorrect calculation introduced in
commit ec5a787d ("Bluetooth: controller: Fix multiple
master role event scheduling") and revert a related
incorrect fix in commit a02606cb ("Bluetooth:
controller: Fix missing ticks to us conversion").

Fixes the controller assert in ctrl.c line number 1477. A
64-bit arithmetic took ~35 us in Radio ISR for nRF51 causing
the ISR to take too much time before packet buffer could be
set.

Also, fixed master scheduling by correctly accounting for
the jitter between each master event.

Relates to: #5486

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