irq.h: fix missing/extra kernel-doc

Impact: fix kernel-doc build

Fix missing & excess irq.h kernel-doc:

Warning(include/linux/irq.h:182): No description found for parameter 'irq'
Warning(include/linux/irq.h:182): Excess struct/union/enum/typedef member 'affinity_entry' description in 'irq_desc'

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>