iio: chemical: sgp30: Drop use of %hx in format string.

Since:
commit cbacb5ab ("docs: printk-formats: Stop encouraging use of
unnecessary %h[xudi] and %hh[xudi]")
use of these format strings has been discouraged.

As there are only a few such uses in IIO, lets clear them all out and
avoid chance of them getting copied into new drivers.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Cc: Nathan Chancellor <nathan@kernel.org>
Cc: Andreas Brauchli <andreas.brauchli@sensirion.com>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Link: https://lore.kernel.org/r/20210517125554.1463156-5-jic23@kernel.org