Bluetooth: GATT: Fix long write procedure

Long write procedure currently requires BT_GATT_PERM_PREPARE_WRITE to
be set otherwise the prepares would fail. This changes the behavior so
that BT_GATT_PERM_PREPARE_WRITE enables checking each prepare chunk
skipping it otherwise.

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