ab8500_charger: Convert to the new USB OTG calls

This patch fixes the following build errors:

ab8500_charger.c: In function 'ab8500_charger_remove':
ab8500_charger.c:2519:2: error: implicit declaration of function 'otg_unregister_notifier' [-Werror=implicit-function-declaration]
ab8500_charger.c:2520:2: error: implicit declaration of function 'otg_put_transceiver' [-Werror=implicit-function-declaration]
ab8500_charger.c: In function 'ab8500_charger_probe':
ab8500_charger.c:2688:2: error: implicit declaration of function 'otg_get_transceiver' [-Werror=implicit-function-declaration]
ab8500_charger.c:2688:10: warning: assignment makes pointer from integer without a cast [enabled by default]
ab8500_charger.c:2695:2: error: implicit declaration of function 'otg_register_notifier' [-Werror=implicit-function-declaration]

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