drivers: i2c_nrfx_twi_rtio: Fix multi-writes with OP_TRANSACTION flag

This patch establishes proper handling of RTIO OPs with multiple write
requests in the same transfer (as in: including OP_TRANSACTION flag).
An example of this is captured in test_ram_rtio_write_with_transaction
test). This patch makes that testcase pass for i2c_nrfx_twi_rtio.

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