atmel_lcdfb: change irq_base definition to allow error reporting

Changed because old the definition of unsigned long cannot be negative.

Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Reported-by: Roel Kluin <roel.kluin@gmail.com>
Cc: Haavard Skinnemoen <hskinnemoen@atmel.com>
Cc: Andrew Victor <linux@maxim.org.za>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>