gigaset: return -ENOTTY for unimplemented functions

A number of functions in the usb_gigaset module will return -EINVAL if
CONFIG_GIGASET_UNDOCREQ is not set. Make these return -ENOTTY as it's
more specific and it might make it easier to see (from userspace) why
these functions actually fail.

Impact: some error return codes changed

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