tests: i2c_ram: rtio: Fix sequence to prepare ops to avoid overwriting

`rtio_sqe_prep_write()` and `rtio_sqe_prep_read()` memset the sqe
struct to zeroes, clearing any configuration previously set. This
commit changes this sequence in the test, such that the sqe's are
prepared, then additional flags/configs applied.

Signed-off-by: Luis Ubieda <luisf@croxel.com>