ab8500_fg: Get rid of 'struct v_to_cap'

The struct is duplicated, plus when used it causes the following
warnings:

  CHECK   drivers/power/ab8500_fg.c
ab8500_fg.c:818:13: warning: incorrect type in assignment (different base types)
ab8500_fg.c:818:13:    expected struct v_to_cap *tbl
ab8500_fg.c:818:13:    got struct abx500_v_to_cap *const v_to_cap_tbl
  CC      drivers/power/ab8500_fg.o
ab8500_fg.c: In function 'ab8500_fg_volt_to_capacity':
ab8500_fg.c:818:6: warning: assignment from incompatible pointer type [enabled by default]

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