ab8500_fg: Fix some build warnings on x86_64

Noticed the following warnings:

  CC      drivers/power/ab8500_fg.o
drivers/power/ab8500_fg.c: In function 'charge_full_store':
drivers/power/ab8500_fg.c:2258:2: warning: format '%d' expects argument of type 'int', but argument 4 has type 'ssize_t' [-Wformat]
drivers/power/ab8500_fg.c: In function ‘charge_now_store’:
drivers/power/ab8500_fg.c:2280:2: warning: format '%d' expects argument of type 'int', but argument 4 has type 'ssize_t' [-Wformat]

This patch fixes the issues.

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