drivers: sensor: clean up zephyr_library calls

In drivers/sensor/CMakeLists.txt, we have various lines like this:

    add_subdirectory_ifdef(CONFIG_FOO foo)

Then drivers/sensor/foo/CMakeLists.txt says:

    zephyr_library()
    zephyr_library_sources_ifdef(CONFIG_FOO foo.c)

This is redundant; the foo/CMakeLists.txt won't be added to the build
system unless CONFIG_FOO=y in the first place, so there's no need for
extra boilerplate testing it again.

Remove all these unnecessary instances in each sensor driver's
CMakeLists.txt using this pattern:

    zephyr_library()
    zephyr_library_sources(foo.c)

In a couple of places, the '.c' extension is missing. Add them in for
consistency when that happens.

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>