pinctrl: ralink: Check for null return of devm_kcalloc

In the Linux kernel, the following vulnerability has been resolved:

pinctrl: ralink: Check for null return of devm_kcalloc

Because of the possible failure of the allocation, data->domains might
be NULL pointer and will cause the dereference of the NULL pointer
later.
Therefore, it might be better to check it and directly return -ENOMEM
without releasing data manually if fails, because the comment of the
devm_kmalloc() says "Memory allocated with this function is
automatically freed on driver detach.".