The return value of the pci_find_(ext_)capability() is either zero or the
position of a capability.  It is never negative.

This patch consolidates the form of check from (pos <= 0) to (!pos).
Signed-off-by: Wei Yang <weiyang@linux.vnet.ibm.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>