hso: Fix for 5 sec timeouts with v2.x firmware

Don't send flow control settings to any port other than the modem port.
Older firmware ignored this request but did sent a reply. Newer firmware just
ignores it without reply and causes a 5 second timeout every time a port
(except for the modem port) is opened or if tiocm settings are changed.

Signed-off-by: Jan Dumon <j.dumon@option.com>
Signed-off-by: David S. Miller <davem@davemloft.net>