vfio-pci: Fix error path sign

This is an impossible error path due to the fact that we're reading a
kernel provided, rather than user provided link, which will certainly
always fit in PATH_MAX.  Currently it returns a fixed 26 char path
plus %d group number, which typically maxes out at double digits.
However, the caller of the initfn certainly expects a less-than zero
return value on error, not just a non-zero value.  Therefore we
should correct the sign here.

Reported-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>