stm32/dma: Remove superfluous asserts from DMA hot path
stm32/dma: Remove superfluous asserts from DMA hot path

This commit removes a number of assert statements that should already
have been hit before. This is the reason that the assert in the
acquire function is left.

OTAkeys/RIOT,RIOT-OS/RIOT,authmillenon/RIOT,RIOT-OS/RIOT,OlegHahm/RIOT,kYc0o/RIOT,authmillenon/RIOT,jasonatran/RIOT,kaspar030/RIOT,ant9000/RIOT,jasonatran/RIOT,kYc0o/RIOT,kYc0o/RIOT,ant9000/RIOT,OTAkeys/RIOT,jasonatran/RIOT,kaspar030/RIOT,basilfx/RIOT,authmillenon/RIOT,authmillenon/RIOT,RIOT-OS/RIOT,OTAkeys/RIOT,miri64/RIOT,kaspar030/RIOT,basilfx/RIOT,kaspar030/RIOT,jasonatran/RIOT,jasonatran/RIOT,ant9000/RIOT,ant9000/RIOT,authmillenon/RIOT,basilfx/RIOT,RIOT-OS/RIOT,kYc0o/RIOT,OlegHahm/RIOT,authmillenon/RIOT,OlegHahm/RIOT,miri64/RIOT,ant9000/RIOT,OlegHahm/RIOT,OTAkeys/RIOT,OlegHahm/RIOT,miri64/RIOT,basilfx/RIOT,OTAkeys/RIOT,miri64/RIOT,miri64/RIOT,RIOT-OS/RIOT,basilfx/RIOT,kYc0o/RIOT,kaspar030/RIOT