rtio: remove padding field _resv0 from struct rtio_sqe

The `_resv0` field is no longer needed after increasing the size of
`iodev_flags` from `uint16_t` to `uint32_t` by this PR zephyrproject-rtos#80177.

Signed-off-by: Pisit Sawangvonganan <pisit@ndrsolution.com>

Author: ndrs-pst

include/zephyr/rtio/rtio.h

@@ -292,8 +292,6 @@ struct rtio_sqe {
 
 	uint32_t iodev_flags; /**< Op iodev flags */
 
-	uint16_t _resv0;
-
 	const struct rtio_iodev *iodev; /**< Device to operation on */
 
 	/**