dmaengine: st_fdma: fix uninitialized variable access

The newly added st_fdma driver introduces a build warning for
allmodconfig when we add '-Wmaybe-uninitialized':

drivers/dma/st_fdma.c: In function 'st_fdma_probe':
drivers/dma/st_fdma.c:777:5: error: 'ret' may be used uninitialized in this function [-Werror=maybe-uninitialized]

The warning is correct, though this can't happen in practice
as the check is redundant (we don't get to this function if
the pointer is NULL). Even if the function were called with a
NULL of_node, the check is not needed because of_property_read_u32
can deal with a NULL argument by returning an error.

Removing the unnecessary code simplifies the function and avoids
the condition that we get the warning for.

Fixes: 6b4cd727 ("dmaengine: st_fdma: Add STMicroelectronics FDMA engine driver support")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Peter Griffin <peter.griffin@linaro.org>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>