Bluetooth: Controller: Fix ticks_current initialization

Fix ticker instance ticks_current initialization to be
acquired from the RTC counter value. The RTC counter may
have run in previous use of ticker before bt_disable hence
keep the instance ticks_current equal to RTC counter value
when ticker_init() is called again.

Relates to commit 4349a475 ("Bluetooth: Controller: Add
deinit() infrastructure").

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