drivers/mailbox/omap: make mbox->irq signed for error handling

There is a bug in omap2_mbox_probe() where we try do:

		mbox->irq = platform_get_irq(pdev, info->irq_id);
		if (mbox->irq < 0) {

The problem is that mbox->irq is unsigned so the error handling doesn't
work.  I've changed it to a signed integer.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Cc: Suman Anna <s-anna@ti.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Omar Ramirez Luna <omar.ramirez@copitl.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>