ARM: OMAP: Remove STACKSIZE for IRQ and FIQ if unused

This patch removes the definition of stack sizes for
irq and fiq if the CONFIG_USE_IRQ is undefined before.

Acked-by: Enric Balletbo i Serra <eballetbo@iseebcn.com>
Acked-by: Tom Rini <trini@ti.com>
Acked-by: Igor Grinberg <grinberg@compulab.co.il>
Signed-off-by: Thomas Weber <weber@corscience.de>
Acked-by: Luca Ceresoli <luca.ceresoli@comelit.it>