Staging: comedi: Remove 2.4 irqreturn_t compatibility in comedi/interrupt.h

This allows the comedi driver to build after commit bedd3..058bb
is applied.

Signed-off-by: Bill Pemberton <wfp5p@virginia.edu>
Reported-by: Sachin Sant <sachinp@in.ibm.com>
Reported-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>