i2c/i3c: rtio: explicit zeroing of SQE flags

Explcitly zero the flags of the SQE obtained from `rtio_sqe_acquire`
when the `rtio_sqe_prep_*` helpers aren't used (the helpers memset the
entire struct to 0).

The flags must be reset to ensure the `RTIO_SQE_TRANSACTION` and
`RTIO_SQE_CHAINED` are not retained from a previous SQE.

Signed-off-by: Jordan Yates <jordan@embeint.com>