Bluetooth: Create unified helper function for updating page scan

Similar to our hci_update_background_scan() function we can simplify a
lot of code by creating a unified helper function for doing page scan
updates. This patch adds such a function to hci_core.c and updates all
the relevant places to use it.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>