staging: sm750fb: fix CamelCase for dispSet var

Fix warning reported by checkpatch:

  CHECK: Avoid CamelCase: <dispSet>

By renaming dispSet to disp_set

Signed-off-by: Petr Sedlak <hugosedlak@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>