gigaset: correct clearing of at_state strings on RING

In RING handling, clear the table of received parameter strings in
a loop like everywhere else, instead of by enumeration which had
already gotten out of sync.

Impact: minor bugfix
Signed-off-by: Tilman Schmidt <tilman@imap.cc>
Acked-by: Karsten Keil <keil@b1-systems.de>
CC: stable@kernel.org
Signed-off-by: David S. Miller <davem@davemloft.net>