staging: echo: move to drivers/misc/

The code is clean, there are users of it, so it doesn't belong in
staging anymore, move it to drivers/misc/.

Cc: Steve Underwood <steveu@coppice.org>
Cc: David Rowe <david@rowetel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>