Staging: echo: remove __cplusplus macro magic

The kernel is written in C, so remove the __cplusplus macro magic from the
driver.

Cc: David Rowe <david@rowetel.com>
Cc: Steve Underwood <steveu@coppice.org>
Signed-off-by: Pekka Enberg <penberg@cs.helsinki.fi>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>