mos7720: bury dead TIOCM... in ->ioctl()

These ioctls never reach driver's ->ioctl() - tty_ioctl() handles
them on its own.  ->tiocm[gs]et() is what actually gets called,
and mos7720 provides those, with results equivalent to what the
unreachable code would be doing when called.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>