flash: spi_nor: fix building on XCC
flash: spi_nor: fix building on XCC

For some reason, XCC fails to build complaining segfault
during CGPREP phase. Adding an assignment to a volatile
return value seems to fix this. This provides an easily
revertable commit to workaround the issue.

Fixes #37734

Signed-off-by: Daniel Leung <d94e04d205b5962d7873a139ef298b03c1717f72@intel.com>

finikorg/zephyr,galak/zephyr,finikorg/zephyr,galak/zephyr,galak/zephyr,galak/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr