TTY: move tty_lookup_driver to switch-cases

The labels express more the nature of the decision tree. We returned
from each if with a driver. Now we do this at the end of the function
and the code flow is clear.

While at it, remove an obsolete comment (we already take the
reference).

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Cc: Alan Cox <alan@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>