regmap: debugfs: Jump to the next readable register

Improve the speed of the loop jumping to the next
available register

Signed-off-by: Lucas Tanure <tanureal@opensource.cirrus.com>
Reviewed-by: Charles Keepax <ckeepax@opensource.cirrus.com>
Tested-by: Charles Keepax <ckeepax@opensource.cirrus.com>
Signed-off-by: Mark Brown <broonie@kernel.org>