Most functions that access the rdev lock the rdev mutex before looking
at data.  ...but not the code that implements the debugfs
regulator_summary.  It probably should though, so let's do it.

Note: this fixes no known issues.  The problem was found only by code
inspection.
Signed-off-by: NDouglas Anderson <dianders@chromium.org>
Signed-off-by: NMark Brown <broonie@kernel.org>