charger-manager: Simplify charger_get_property(), get rid of a warning

This patch fixes the following warning and makes the code a little
bit more simpler:

  CC      drivers/power/charger-manager.o
charger-manager.c: In function 'charger_get_property':
charger-manager.c:429:15: warning: 'i' may be used uninitialized in this function [-Wuninitialized]

Signed-off-by: Anton Vorontsov <anton.vorontsov@linaro.org>