ab8500: Turn unneeded global symbols into local ones

The patch fixes the following sparse warning:

drivers/power/ab8500_charger.c:1619:6: warning: symbol 'ab8500_charger_detect_usb_type_work' was not declared. Should it be static?
drivers/power/abx500_chargalg.c:1709:24: warning: symbol 'abx500_chargalg_sysfs_ops' was not declared. Should it be static?
drivers/power/ab8500_fg.c:2328:24: warning: symbol 'ab8500_fg_sysfs_ops' was not declared. Should it be static?

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