iio: bh1780: dereference the client properly

The code in runtime_[suspend|resume] was assuming that the
i2c client data was the bh1780 state container, but it contains
the IIO device. So first dereference the IIO device from the
i2c client, then get the state container using the iio_priv()
call.

Fixes: 1f0477f1 ("iio: light: new driver for the ROHM BH1780")
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>