fix test failed issue while rpmsg is running (Bugfix)

[stanley31huang]

fix test failed issue while rpmsg is running by default

Description

the rpmsg_load_firmware.py scripts would crashed with following situation.

1. when the rpmsg is enabled by default, the state would be running
2. the state can be configure as "start" or "stop" only.

So this PR is trying to addressed these two issues.

Resolved issues

None

Documentation

https://www.kernel.org/doc/Documentation/ABI/testing/sysfs-class-remoteproc

Tests

iotuc@ubuntu:~$ sudo checkbox-shiner.shell 
checkbox-shiner runtime shell, type 'exit' to quit the session
root@ubuntu:/home/iotuc# python3 /var/tmp/checkbox-providers/checkbox-provider-ce-oem/bin/rpmsg_load_firmware.py test-reload --device remoteproc0 --file rpmsg_lite_str_echo_rtos_imxcm4.elf --path /home/iotuc
2025-01-22 05:50:17 INFO     # Start load M4 firmware test
2025-01-22 05:50:17 INFO     Stop the Remote processor
2025-01-22 05:50:17 INFO     Configure the firmware file to rpmsg_lite_str_echo_rtos_imxcm4.elf and firmware path to /home/iotuc
2025-01-22 05:50:17 INFO     Start the Remote processor
2025-01-22 05:50:17 INFO     # start time: 1737525017.3082879
2025-01-22 05:50:17 INFO     Validate RPMSG related log from journal logs
2025-01-22 05:50:17 INFO     start stage: remoteproc remoteproc0: powering up imx-rproc
2025-01-22 05:50:17 INFO     boot_image stage: remoteproc remoteproc0: Booting fw image rpmsg_lite_str_echo_rtos_imxcm4.elf, size 158324
2025-01-22 05:50:17 INFO     ready stage: remoteproc remoteproc0: remote processor imx-rproc is now up
2025-01-22 05:50:17 INFO     # Reload M4 firmware successful

[rickwu666666]

LGTM