sparc: use scnprintf() in show_pciobppath_attr() in vio.c

snprintf() returns the number of bytes that would be written,
which may be greater than the the actual length to be written.

show_pciobppath_attr() should return the number of bytes printed
into the buffer. This is the return value of scnprintf().

Signed-off-by: Chen Zhou <chenzhou10@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>