dmaengine: stm32-mdma: Remove dead code in stm32_mdma_irq_handler()

Local variable chan is initialized by an address of element of chan array
that is part of stm32_mdma_device struct, so it does not make sense to
compare chan with NULL.

Found by Linux Verification Center (linuxtesting.org) with SVACE.

Signed-off-by: Alexey Khoroshilov <khoroshilov@ispras.ru>
Fixes: a4ffb13c ("dmaengine: Add STM32 MDMA driver")
Reviewed-by: Amelie Delaunay <amelie.delaunay@foss.st.com>
Link: https://lore.kernel.org/r/1655072638-9103-1-git-send-email-khoroshilov@ispras.ru


Signed-off-by: Vinod Koul <vkoul@kernel.org>