CVE-2021-47159 net: dsa: fix a crash if ->get_sset_count() fails

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

net: dsa: fix a crash if ->get_sset_count() fails

If ds->ops->get_sset_count() fails then it “count” is a negative error
code such as -EOPNOTSUPP. Because “i” is an unsigned int, the negative
error code is type promoted to a very high value and the loop will
corrupt memory until the system crashes.

Fix this by checking for error codes and changing the type of “i” to
just int.