USB: remove references to port->port.count from the serial drivers

This patch (as1344) removes references to port->port.count from the
USB serial drivers.  Now that serial ports are properly reference
counted, port.count checking is unnecessary and incorrect.  Drivers
should assume that the port is in use from the time the open method
runs until the close method is called.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>