iio: temperature: ltc2983: Use local device pointer consistently

iio: temperature: ltc2983: Use local device pointer consistently

Some functions define a local 'dev' pointer but still use bare
'&st->spi->dev' in some code paths, and some don't have it at all.
Replace bare references with the local pointer for consistency and
collapse some wrapped lines that now fit within 80 characters.

Reviewed-by: Joshua Crofts <joshua.crofts1@gmail.com>
Signed-off-by: Liviu Stan <liviu.stan@analog.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>