USB: OHCI: fix build error related to ohci_suspend/resume

USB: OHCI: fix build error related to ohci_suspend/resume

Commit 9a11899c5e69 (USB: OHCI: add missing PCI PM callbacks to
ohci-pci.c) added missing ohci_suspend and ohci_resume callback
pointers, but forgot that these callbacks are declared and defined
only when CONFIG_PM is enabled.

This patch adds a preprocessor conditional to avoid build errors when
PM is disabled.

Reported-by: Guenter Roeck <linux@roeck-us.net>
Tested-by: Guenter Roeck <linux@roeck-us.net>
Reported-by: Meelis Roos <mroos@linux.ee>,
Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>