usb: usb251xb: Add 5,6,7 ports boost settings

USB electrical signaling drive strength boost bit is also supported
by USB2517 hub. Since it got three addition ports, the designers
needed to add one more register for initialization. It turned out
to be formerly reserved 0xF7. As before we just initialize it with
default zeros.

Signed-off-by: Serge Semin <fancer.lancer@gmail.com>
Acked-by: Richard Leitner <richard.leitner@skidata.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>