Bluetooth: Controller: Update to increase simultaneous connection limit

Update Controller to use uint16_t instead of uint8_t to
allow buffer counts to exceed beyond 255. Also fix pointer
arithmetic from being restricted to uint16_t that prevented
calculating the memory index for large offsets. This will
now allow support for 250 simultaneous connections, limited
by ticker_id type being uint8_t.

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