gigaset: fix format string typo in CAPI dial command

A missing dot lead to garbage characters being included in the
dial command generated from a CAPI CONNECT_REQ message, which
interestingly enough worked anyway, illustrating the resilience
of the device.

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