comgt: Allow using non-TTY devices

Some Huawei mobile broadband sticks utilizing the NCM protocol expose
the control channel as a cdc-wdm device node instead of a virtual TTY.
This device node does not support the terminal ioctls. This patch
adds a check whether the provided device is a TTY or not and does not
attempt to use the terminal ioctls if they are not supported.

v2: reduce diffstat by simplifying code a little
Signed-off-by: Matti Laakso <malaakso@elisanet.fi>
SVN-Revision: 44054