PCI: fix bogus "'device' may be used uninitialized" warning in pci_slot

I get warnings about 'device' possibly being used uninitialised.  While
I can deduce this is not true, it seems that GCC can't.  This patch
changes `check_slot' to return device on success and -1 on error, which
shuts GCC up.

Acked-by: Alex Chiang <achiang@hp.com>
Signed-off-by: Matthew Wilcox <willy@linux.intel.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>