TTY: crisv10, remove unused tmp_buf

This used to be a helper buffer for generic_serial. generic_serial is
gone, tmp_buf shall be removed.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Cc: Mikael Starvik <starvik@axis.com>
Acked-by: Jesper Nilsson <jesper.nilsson@axis.com>
Cc: linux-cris-kernel@axis.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>