USB: serial: ssu100: fix control-message error handling

Make sure to detect short control-message transfers rather than continue
with zero-initialised data when retrieving modem status and during
device initialisation.

Fixes: 52af9545

 ("USB: add USB serial ssu100 driver")
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Johan Hovold <johan@kernel.org>