gigaset: correct range checking off by one error

Correct a potential array overrun due to an off by one error in the
range check on the CAPI CONNECT_REQ CIPValue parameter.
Found and reported by Dan Carpenter using smatch.

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