isdn/gigaset: Use kzalloc instead of open-coded field zeroing

This replaces a kmalloc followed by a bunch of per-field zeroing with a
single kzalloc call, reducing the lines of code.

Cc: Paul Bolle <pebolle@tiscali.nl>
Cc: Karsten Keil <isdn@linux-pingi.de>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Johan Hovold <johan@kernel.org>
Cc: gigaset307x-common@lists.sourceforge.net
Cc: netdev@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: David S. Miller <davem@davemloft.net>