[ARM] 4872/1: Replaces buggy macro in S3C2410 irq include

This is a bug correction for a macro that generated wrong results.
Nobody used it in official kernel tree, my driver did.

Signed-off-by: Davide Rizzo <davide@elpa.it>
Acked-by: Ben Dooks <ben-linux@fluff.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>