ab8500_fg: Overflow in current calculation

When calculating the average current the nominator will
overflow when the charging current is high.

Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Henrik Sölver <henrik.solver@stericsson.com>
Reviewed-by: Par-Olof HAKANSSON <par-olof.hakansson@stericsson.com>
Reviewed-by: Jonas ABERG <jonas.aberg@stericsson.com>
Tested-by: Par-Olof HAKANSSON <par-olof.hakansson@stericsson.com>
Signed-off-by: Anton Vorontsov <anton@enomsg.org>