Bluetooth: Audio: Fix NULL pointer ref in bt_csis_lock

bt_csis_lock would call write_set_lock with `attr` set to
NULL, which was dereferenced to get the bt_csis instance.

This commit modified the functions such that we instead have
common function to handle changes to the lock value, called
from either the GATT write callback function, or the
bt_csis_lock, so that we have a clearer separation of what
the individual functions do.

Signed-off-by: Emil Gydesen <emil.gydesen@nordicsemi.no>