fbdev: omap2: fix warnings in dss core

Commit 60d2fa0d ("fbdev: omap2: no need to check return value of
debugfs_create functions") changed the declaration of the return value
of function dss_debugfs_create_file() and the following two warnings
appeared:

drivers/video/fbdev/omap2/omapfb/dss/core.c: In function ‘dss_debugfs_create_file’:
drivers/video/fbdev/omap2/omapfb/dss/core.c:139:9: warning: ‘return’ with a value,
 in function returning void
  return 0;
         ^
drivers/video/fbdev/omap2/omapfb/dss/core.c: In function ‘omap_dss_probe’:
drivers/video/fbdev/omap2/omapfb/dss/core.c:172:6: warning: unused variable ‘r’
 [-Wunused-variable]
  int r;
      ^

Rework so function dss_debugfs_create_file() that is declared to return
void don't 'return 0' and remove the declaration of the unused variable
'r'.

Signed-off-by: Anders Roxell <anders.roxell@linaro.org>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>