staging/comedi: fix build for USB not enabled

Calls to optional subsystems cannot be made indiscriminately.
Enclose all of the usb helper functions inside
#if IS_ENABLED(CONFIG_USB)
to fix these build errors.

(The pci helper functions are OK since there are stubs in
linux/pci.h for the called functions when PCI is not enabled.
Possibly the same could be done for the called USB functions.)

ERROR: "usb_deregister" [drivers/staging/comedi/comedi.ko] undefined!
ERROR: "usb_register_driver" [drivers/staging/comedi/comedi.ko] undefined!

Signed-off-by: Randy Dunlap <rdunlap@xenotime.net>
Cc: Ian Abbott <abbotti@mev.co.uk>
Cc: Frank Mori Hess <fmhess@users.sourceforge.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>