drivers: dma: stm32: Use volatile for flag used in ISR
drivers: dma: stm32: Use volatile for flag used in ISR

Use volatile for flag shared between normal code and ISR to
prevent the compiler from possibly optimizing it away.

Signed-off-by: Erwin Rol <0c2a6e12a3ee3123563f88831cd6d692c33fb195@erwinrol.com>

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