tests: spi_loopback: Extend RTIO fallback messages to 5

Loopback test requires this in order to properly execute half
transactions. Otherwise, testsuite fails.

It was overlooked when fixing #85894.

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

Author: ubieda

tests/drivers/spi/spi_loopback/testcase.yaml

@@ -30,6 +30,7 @@ tests:
   drivers.spi.loopback.rtio:
     extra_configs:
       - CONFIG_SPI_RTIO=y
+      - CONFIG_SPI_RTIO_FALLBACK_MSGS=5
     platform_allow:
       - robokit1
       - mimxrt1170_evk/mimxrt1176/cm7