staging: sm750fb: Rename miniDiff

Rename miniDiff to mini_diff to avoid CamelCase. Problem found using
checkpatch.pl
CHECK: Avoid CamelCase: <miniDiff>

Signed-off-by: Amitoj Kaur Chawla <amitoj1606@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>