mtd: rawnand: fsmc: Fix error code in fsmc_nand_probe()

If dma_request_channel() fails then the probe fails and it should
return a negative error code, but currently it returns success.

fixes: 4774fb0a ("mtd: nand/fsmc: Add DMA support")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/YCqaOZ83OvPOzLwh@mwanda