iio: si7020: Swap data byte order

The Silicon Labs Si7013, Si7020, and Si7021 family of I2C humidity and
temperature sensors deliver 16 bit data high byte first.

See the datasheet available at:
https://www.silabs.com/Support%20Documents%2fTechnicalDocs%2fSi7020-A20.pdf



But as documented in Documentation/i2c/smbus-protocol,
i2c_smbus_read_word_data() expects the low byte first.

Change the driver to use i2c_smbus_read_word_swapped to get correct byte
order.

Signed-off-by: Chris Lesiak <chris.lesiak@licor.com>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>