regmap: clean up debugfs if regmap_init fails

If debugfs isn't cleaned up, stale files will be left in the filesystem
which will cause an OOPS when accessed the first time, and hang the
accessing application when accessed again, presumably due to some lock
being left held.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>