isdn/gigaset: avoid copying AT commands twice

Change the Gigaset driver's internal write_cmd interface to accept a
cmdbuf structure instead of a string. This avoids copying formatted
AT commands a second time.

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