iio: pressure-core: st: Expand and rename LPS331AP's channel descriptor

Due to the MACRO used, the task of reading, understanding and maintaining
the LPS331AP's channel descriptor is substantially difficult. This patch
is based on the view that it's better to have easy to read, maintainable
code than to save a few lines here and there. For that reason we're
expanding the array so initialisation is completed in full.

Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>