dmaengine: st_fdma: Fix the error return code in st_fdma_probe()

In case of error, the function st_slim_rproc_alloc() returns ERR_PTR()
and never returns NULL. The NULL test in the return value check should
be replaced with IS_ERR().

Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Acked-by: Peter Griffin <peter.griffin@linaro.org>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>