spi_rtio: fix transactions for default handler

This patch fixes transaction op items not performed within a single
SPI transfer. This is common for Write + Read commands, that depend on
the CS kept asserted until the end, otherwise the context will be lost.
A similar fix was applied to i2c_rtio_default on #79890.

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