ab8500_fg: Get rid of 'struct battery_type'

The struct is duplicated, plus causes the following flood:

  CC      drivers/power/ab8500_fg.o
ab8500_fg.c: In function ‘ab8500_fg_get_ext_psy_data’:
b8500_fg.c:2081:8: warning: assignment from incompatible pointer type [enabled by default]

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