ACPI / PCI: pci_slot: Use generic pr_debug utility

Remove the dbg macro and debug module parameter and use
the generic kernel facility.

Trivially reduces defconfig object size on x86-64

$ size drivers/acpi/pci_slot.o*
   text	   data	    bss	    dec	    hex	filename
    880	    752	      4	   1636	    664	drivers/acpi/pci_slot.o.new
    935	    752	      5	   1692	    69c	drivers/acpi/pci_slot.o.old

Signed-off-by: Joe Perches <joe@perches.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>