da9052-battery: Fix da9052_determine_vc_tbl_index's return value

The patch fixes the following warnings:

  CHECK   drivers/power/da9052-battery.c
drivers/power/da9052-battery.c:330:15: warning: symbol 'da9052_determine_vc_tbl_index' was not declared. Should it be static?
  CC      drivers/power/da9052-battery.o
drivers/power/da9052-battery.c: In function 'da9052_determine_vc_tbl_index':
drivers/power/da9052-battery.c:348:1: warning: control reaches end of non-void function [-Wreturn-type]

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