USB: ir-usb: fix set_termios race

Use dynamically allocated urb for baudrate changes rather than
unconditionally submitting the port write urb which may already be in
use.

Compile-only tested.

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