alpha: nautilus - fix compile failure with gcc-4.3

init_srm_irq() deals with irq's #16 and above, but size of irq_desc
array on nautilus and some other system types is 16. So gcc-4.3
complains that "array subscript is above array bounds", even though
this function is never called on those systems.

This adds a check for NR_IRQS <= 16, which effectively optimizes
init_srm_irq() code away on problematic platforms.

Thanks to Daniel Drake <dsd@gentoo.org> for detailed analysis
of the problem.

Signed-off-by: Ivan Kokshaysky <ink@jurassic.park.msu.ru>
Cc: Richard Henderson <rth@twiddle.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>