asic3: platform_get_irq() may return signed unnoticed

asic->irq_nr is unsigned. platform_get_irq() may return signed unnoticed

Signed-off-by: Roel Kluin <12o3l@tiscali.nl>
Cc: Joe Perches <joe@perches.com>
Acked-by: Samuel Ortiz <sameo@openedhand.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>