lp8727_charger: Clean up lp8727_charger_changed()

Declare a variable at one line. Just return when no charger exists to make
code simpler.

Signed-off-by: Milo(Woogyom) Kim <milo.kim@ti.com>
Signed-off-by: Anton Vorontsov <anton.vorontsov@linaro.org>