regulator: da9062: Return REGULATOR_MODE_INVALID for invalid mode

-EINVAL is not a valid return value for .of_map_mode, return
REGULATOR_MODE_INVALID instead.

Fixes: 844e7492

 ("regulator: da9062: add of_map_mode support for bucks")
Signed-off-by: Axel Lin <axel.lin@ingics.com>
Link: https://lore.kernel.org/r/20191122045154.802-1-axel.lin@ingics.com
Signed-off-by: Mark Brown <broonie@kernel.org>