iio: use I2C_FUNC_SMBUS_I2C_BLOCK macro

I do not see why the driver requires I2C_FUNC_SMBUS_I2C_BLOCK,
actually only single bytes or words are transferred

Signed-off-by: Peter Meerwald <pmeerw@pmeerw.net>
Cc: Ge Gao <ggao@invensense.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>