usb: usb251xb: Add battery enable setting flag

Battery charging settings are supported by USB251xb hubs only.
USB2517i isn't one of them. So we need to reflect it within the
device-specific data structure. The driver doesn't support dts
property changing this setting, but instead defaults it with zero.
So the flag isn't used anywhere in the driver, but still can be helpful
in future, when necessity of the corresponding dts setting arises.

Signed-off-by: Serge Semin <fancer.lancer@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>