iio:chemical:sgp30: Use local variable dev to simplify code

This cleans up the code at bit, but is primarily here as a precusor
to the next patch. I've only done this for the two functions
which use the dev pointer repeatedly.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Cc: Andreas Brauchli <andreas.brauchli@sensirion.com>
Link: https://lore.kernel.org/r/20200910173242.621168-27-jic23@kernel.org