isdn/gigaset: fix leaks in error path

Take care to free all previously allocated ressources in the
"out of memory" error path of the ISDN_CMD_DIAL branch.
Based on an original patch by Dan Carpenter.

Impact: bugfix
Reported-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: Tilman Schmidt <tilman@imap.cc>
Acked-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>