i2c: mt65xx: fix IRQ check

Iff platform_get_irq() returns 0, the driver's probe() method will return 0
early (as if the method's call was successful).  Let's consider IRQ0 valid
for simplicity -- devm_request_irq() can always override that decision...

Fixes: ce38815d ("I2C: mediatek: Add driver for MediaTek I2C controller")
Signed-off-by: Sergey Shtylyov <s.shtylyov@omprussia.ru>
Reviewed-by: Qii Wang <qii.wang@mediatek.com>
Signed-off-by: Wolfram Sang <wsa@kernel.org>