USB: serial: ch341: fix type promotion bug in ch341_control_in()

The "r" variable is an int and "bufsize" is an unsigned int so the
comparison is type promoted to unsigned.  If usb_control_msg() returns a
negative that is treated as a high positive value and the error handling
doesn't work.

Fixes: 2d5a9c72 ("USB: serial: ch341: fix control-message error handling")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Johan Hovold <johan@kernel.org>