regulator: Add regulator_err2notif() helper

Help drivers avoid storing both supported notification and supported error
flags by supporting conversion from regulator error to notification.
This may help saving some bytes.

Add helper for finding the regulator notification corresponding to a
regulator error.

Signed-off-by: Matti Vaittinen <matti.vaittinen@fi.rohmeurope.com>
Link: https://lore.kernel.org/r/eb1755ac0569ff07ffa466cf8912c6fd50e7c7c6.1637736436.git.matti.vaittinen@fi.rohmeurope.com


Signed-off-by: Mark Brown <broonie@kernel.org>