[PATCH -next] comedi: fix build when USB is not enabled

[Randy Dunlap <rdunlap@xenotime.net>]

From: Randy Dunlap <rdunlap@xenotime.net>

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>
---
 drivers/staging/comedi/drivers.c |    5 +++++
 1 file changed, 5 insertions(+)

--- linux-next-20120518.orig/drivers/staging/comedi/drivers.c
+++ linux-next-20120518/drivers/staging/comedi/drivers.c
@@ -30,6 +30,7 @@
 #include <linux/pci.h>
 #include <linux/usb.h>
 #include <linux/errno.h>
+#include <linux/kconfig.h>
 #include <linux/kernel.h>
 #include <linux/sched.h>
 #include <linux/fcntl.h>
@@ -981,6 +982,8 @@ void comedi_pci_driver_unregister(struct
 }
 EXPORT_SYMBOL_GPL(comedi_pci_driver_unregister);
 
+#if IS_ENABLED(CONFIG_USB)
+
 static int comedi_old_usb_auto_config(struct usb_interface *intf,
 				      struct comedi_driver *driver)
 {
@@ -1043,3 +1046,5 @@ void comedi_usb_driver_unregister(struct
 	comedi_driver_unregister(comedi_driver);
 }
 EXPORT_SYMBOL_GPL(comedi_usb_driver_unregister);
+
+#endif