staging: comedi: conditionally build in USB driver support

Separate the comedi_usb_* functions out of drivers.c into a new
source file, comedi_usb.c. This allows conditionally building
support for comedi USB drivers into the comedi core without the
need for the #if'defery. Fix the Kconfig and Makefile appropriately.
For aesthetic reasons, add some whitespace to the Makefile to keep
everything lined up.

Group all the comedi_usb_* prototypes into one place in comedidev.h.
Protect these prototypes with an #ifdef so that building a comedi
usb driver without USB support will cause a build error. This will
normally not happen as long as the comedi USB driver is placed in
the proper group in the Kconfig.

Remove the #include<linux/usb.h> from comedidev.h and drivers.c. This
include is only needed by the comedi USB driver support code and the
USB drivers. The include should occur in those files.

Removing the include of usb.h exposed a couple drivers that need
<linux/interrupt.h> and <linux/sched.h>. Add the missing includes.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Cc: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>