usb: dwc3: pci: avoid build warning

dwc3_pci_dsm() is only needed if (PM || PM_SLEEP),
we should make sure it's not defined if neither of
those is defined.

This fixes a randconfig build warning.

Acked-by: Arnd Bergmann <arnd@arndb.de>
Reported-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>