sensors: Add missing const to `sensor_read_async_mempool`

Add missing const on `struct rtio_iodev *iodev`
The blocking version, sensor_read(), as well as
rtio_sqe_prep_read_with_pool() which accepts
struct rtio_iodev are both marked as const.
This function should also be marked const.

Signed-off-by: Anthony Williams <anthony289478@gmail.com>