staging: oplc_dcon: Fix compilation warning.

Fix compilation warning:
drivers/staging/olpc_dcon/olpc_dcon.c: In function ‘dcon_probe’:
drivers/staging/olpc_dcon/olpc_dcon.c:704:21: warning: ignoring
return value of ‘device_create_file’, declared with attribute warn_unused_result

and add cleaning of created files when creation of one failed.

Signed-off-by: Marek Belisko <marek.belisko@open-nandra.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>