treewide: simplify line lookup

treewide: simplify line lookup

We're preparing to entirely remove the line objects from the API and
split their functionality between two new objects: line_info and
line_request. The lookup functions must be limited in the process.

This reworks all the find_line methods to: a) always assume that names
looked for are unique within a single chip (because while it's
technically possible for GPIO line names to be non-unique, it doesn't
make sense to look for two lines named the same) and b) return the
hardware offset within the chip instead of the line object.

Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>