Fix IRQ handle bug when interrupt mode.

The driver was incorrectly handled and returned IRQ_HANDLED
while the device is not generating the interrupt.
It happened due to faulty determination of interrupt status register.

Found by: "Ethan" <ethanhsiao@jmicron.com>
Fixed by: "akeemting" <akeem@jmicron.com>
Signed-off-by: Guo-Fu Tseng <cooldavid@cooldavid.org>
Signed-off-by: David S. Miller <davem@davemloft.net>