drivers/dma: stm32 fix dmamux request id valid check
drivers/dma: stm32 fix dmamux request id valid check

The request id is given by the DMA request MUX id which start at offset
1 and are vaid until req_nb + gen_nb.

Signed-off-by: Thomas Stranger <289fb7da2789c69f49f06a4b4e006530cf91f1de@outlook.com>

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