isdn/gigaset: internal function name cleanup

Functions clear_at_state and free_strings did the same thing;
drop one of them, keeping the more descriptive name.
Drop a redundant call.
Rename function dealloc_at_states to dealloc_temp_at_states
to clarify its purpose.

Signed-off-by: Tilman Schmidt <tilman@imap.cc>
Signed-off-by: David S. Miller <davem@davemloft.net>