dmaengine: stm32-dmamux: Fix a NULL vs IS_ERR() check in probe

devm_ioremap_resource() doesn't return NULL, it returns error pointers.

Fixes: df7e762d ("dmaengine: Add STM32 DMAMUX driver")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Pierre-Yves MORDRET <pierre-yves.mordret@st.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>