interrupt.h: fix fatal kernel-doc error

Fix kernel-doc fatal error:
/** beginning a non-kernel-doc comment block:
(That alone does not kill kernel-doc, but the 'enum' was
totally confusing to it.)

Error(/lnx/src/TMP/linux-2.6.34-git6//include/linux/interrupt.h:88): cannot understand prototype: 'enum '
make[2]: *** [Documentation/DocBook/genericirq.xml] Error 1

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>