USB: serial: ark3116: remove dummy TIOCSSERIAL ioctl

USB: serial: ark3116: remove dummy TIOCSSERIAL ioctl

The patch removes unused TIOCSSERIAL ioctl case and adds the default block
to the switch. This will make the ioctl return -ENOTTY to user space (e.g.
setserial), which indicates that TIOCSSERIAL really isn't supported for
these devices currently.

Note that these (dummy) ioctl implementations where added by commit
2541f0ce8fc0 ("USB: ark3116: Add TIOCGSERIAL and TIOCSSERIAL ioctl
calls.") back in 2006. This in turn appears to have been triggered by a
change in a user space tool, wvdial, which started erroring out if
either was missing.

There are some bug reports about that against wvdial from around that
time, and looking at the wvstreams (library) code now, it looks like the
issue has indeed been resolved by handling errors more gracefully (e.g.
just logging them).

User space really should not make assumptions about these ioctl always
being implemented, but if this turns out to be a problem for anyone
using this driver, we'll add TIOCSSERIAL back in some form.

Signed-off-by: Mikhail Zaytsev <flashed@mail.ru>
[johan: amend commit message with backstory ]
Signed-off-by: Johan Hovold <johan@kernel.org>