Correct a PCI enumeration bug introduced in r264011

Ensure that first_func is set to 0 on every iteration of the PCI slot
enumeration loop after the first.  There is a continue statement that would
cause first_func to stay at 1 any PCI device where slot 0 has no functions
until we find a slot that does have a function.  This would cause us to
not enumerate the first PCI function on the device.

Credit to markj@ for spotting the bug.

X-MFC-With: r264011