gpio: sa1100: fix irq probing for ucb1x00

ucb1x00 has used IRQ probing since it's dawn to find the GPIO interrupt
that it's connected to.  However, commit 23393d49 ("gpio: kill off
set_irq_flags usage") broke this by disabling IRQ probing on GPIO
interrupts.  Fix this.

Fixes: 23393d49 ("gpio: kill off set_irq_flags usage")
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>