iio: adc: vf610: fix the adc register read fail issue

Read the register only when the adc register address is 4 byte aligned.
(rather than the other way around).

Signed-off-by: Haibo Chen <haibo.chen@freescale.com>
Signed-off-by: Fugang Duan <B38611@freescale.com>
Cc: <Stable.vger.kernel.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>