ARM: fix bug of checking on signed return value using unsigned statement in w90p910 platform

To fix the bug of checking on signed return value using unsigned statement.
Thanks Roel Kluin for digging out it.

Signed-off-by: Roel Kluin <roel.kluin@gmail.com>
Signed-off-by: Wan ZongShun <mcuos.com@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>