ser_gigaset: fix deallocation of platform device structure

When shutting down the device, the struct ser_cardstate must not be
kfree()d immediately after the call to platform_device_unregister()
since the embedded struct platform_device is still in use.
Move the kfree() call to the release method instead.

Signed-off-by: Tilman Schmidt <tilman@imap.cc>
Fixes: 2869b23e

 ("drivers/isdn/gigaset: new M101 driver (v2)")
Reported-by: Sasha Levin <sasha.levin@oracle.com>
Signed-off-by: Paul Bolle <pebolle@tiscali.nl>
Signed-off-by: David S. Miller <davem@davemloft.net>