mmc: sunplus: Fix platform_get_irq() error checking

The platform_get_irq() function returns negative error codes on failure.

Signed-off-by: Harshit Mogalapalli <harshit.m.mogalapalli@oracle.com>
Reviewed-by: Dan Carpenter <dan.carpenter@linaro.org>
Link: https://lore.kernel.org/r/20230809071812.547229-2-harshit.m.mogalapalli@oracle.com
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>