USB: mos7720: remove incorrect read-urb check

Remove incorrect and unnecessary check for port->read_urb which is not
set to NULL, contrary to what seems to be assumed, when urb is killed.

Compile only-tested.

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