Staging: comedi: fix Kbuild

comedi doesn't like being built into the kernel right now, so force it
to be a module.
Reported-by: NKamalesh Babulal <kamalesh@linux.vnet.ibm.com>
Reported-by: NRandy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@suse.de>

drivers/staging/comedi/Kconfig

 config COMEDI
 	tristate "Data Acquision support (comedi)"
 	default N
+	depends on m
 	---help---
 	  Enable support a wide range of data acquision devices
 	  for Linux.