regulator: core: Add locking to debugfs regulator_summary

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: Douglas Anderson <dianders@chromium.org>
Signed-off-by: Mark Brown <broonie@kernel.org>