greybus: checkpatch.pl: Mark --no-tree as default option

Don't know why, but checkpatch checks if we are running it from top of a
kernel tree or not, but then it also provides an option to suppress the
warning using --no-tree.

Instead of forcing everyone to use this every time, lets make this
behavior default.

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>