Bluetooth: controller: Fix ticker to use u32_t ticks_slot

ticker timespace reservation can range up to 10.24 seconds,
needing 19-bits to represent in 32KHz clock units. Hence,
fix controller implementation to use u32_t to store ticks
slot values.

Without this fix, the controller is asserting in scan_adv
sample when using continuous scanning with 2 second interval
and window.

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