Bluetooth: Controller: Fix building ticker without remainder support

A few things were not flagged by CONFIG_BT_TICKER_REMAINDER_SUPPORT
that should have been; This caused building without it to fail

Initialize remainder to 0 where used with ticker_next_slot_get_ext(),
since it may not get set if remainder support is off

Signed-off-by: Troels Nilsson <trnn@demant.com>