dmaengine: stm32-dma: Fix unsigned variable compared with zero

Commit f4fd2ec0: ("dmaengine: stm32-dma: use platform_get_irq()") used
unsigned variable irq to store the results and check later for negative
errors, so update the code to use signed variable for this

Fixes: f4fd2ec0

 ("dmaengine: stm32-dma: use platform_get_irq()")
Reported-by: kbuild test robot <lkp@intel.com>
Reported-by: Julia Lawall <julia.lawall@lip6.fr>
Acked-by: Julia Lawall <julia.lawall@lip6.fr>
Signed-off-by: Vinod Koul <vkoul@kernel.org>