isdn/gigaset: Provide cardstate context for bas timer callbacks

While the work callback uses the urb to find cardstate from bas_cardstate,
this may not be valid for timer callbacks. Instead, introduce a direct
pointer back to the cardstate from bas_cardstate for use in timer
callbacks.

Reported-by: Paul Bolle <pebolle@tiscali.nl>
Fixes: 4cfea08e ("isdn/gigaset: Convert timers to use timer_setup()")
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>