MIPS: ralink: Fix platform_get_irq's error checking

MIPS: ralink: Fix platform_get_irq's error checking

The platform_get_irq() function returns negative if an error occurs.
zero or positive number on success. platform_get_irq() error checking
for zero is not correct.

Signed-off-by: Arvind Yadav <arvind.yadav.cs@gmail.com>
Cc: john@phrozen.org
Cc: linux-kernel@vger.kernel.org
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/17783/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>