Bluetooth: GATT: Make bt_gatt_write take a struct

This makes bt_gatt_write similar to bt_gatt_read where the parameters are
stored in a struct which can be used to store intermediate values while
the operation is in progress.

Change-Id: I3c62af137b99985690cf88dcc37a977a0be891f5
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>