staging: olpc_dcon: fix sparse symbol not declared warning

This patch gets rid of the following sparse warning:

drivers/staging/olpc_dcon/olpc_dcon.c:787:19: warning: symbol 'dcon_driver' was not declared. Should it be static?

Signed-off-by: Murilo Opsfelder Araujo <mopsfelder@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>