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

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

BugLink: http://bugs.launchpad.net/bugs/1691418
commit 1eac5c244f705182d1552a53e2f74e2775ed95d6 upstream.

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

Fixes: 52af95459939 ("USB: add USB serial ssu100 driver")
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>