Bluetooth: GATT: Add flags to attribute write callback

This adds flags parameter to write callback which can be used to indicate
that data only need to be prepared with use of BT_GATT_WRITE_FLAG_PREPARE
fixing qualification tests that needs to check authorization or other
errors that cannot be verified with just the permissions.

Change-Id: I3d662b2027718ffb52a280e3bbc9750be14f89ae
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>