USB: mos7720: fix hardware flow control

commit a26f009a070e840fadacb91013b2391ba7ab6cc2 upstream.

The register access to enable hardware flow control depends on the
device port number and not the port minor number.

Signed-off-by: Johan Hovold <jhovold@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>