staging: gigaset: fix illegal free on probe errors

The driver failed to initialise its receive-buffer pointer, something
which could lead to an illegal free on late probe errors.

Fix this by making sure to clear all driver data at allocation.

Fixes: 2032e2c2 ("usb_gigaset: code cleanup")
Cc: stable <stable@vger.kernel.org>     # 2.6.33
Cc: Tilman Schmidt <tilman@imap.cc>
Signed-off-by: Johan Hovold <johan@kernel.org>
Link: https://lore.kernel.org/r/20191202085610.12719-3-johan@kernel.org


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>