EDAC/synopsys: Return the correct value in mc_probe()

Return the error value if the inject sysfs file creation fails, rather
than returning 0, to signal to the upper layer that the ->probe function
failed.

 [ bp: Massage. ]

Signed-off-by: Zhang Xiaoxu <zhangxiaoxu5@huawei.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Michal Simek <michal.simek@xilinx.com>
Link: https://lkml.kernel.org/r/20201116135810.3130845-1-zhangxiaoxu5@huawei.com