Bluetooth: GATT: Add write struct parameter to its callback

This makes bt_gatt_write consistent with the rest of the API where the
parameters are passed back to its callback.

Change-Id: Ie94208aa661d3620d0cbc5be4a4fb5b3c3ef061c
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>