regulator: handle different error codes in regulator_set_enable_if_allowed

The regulator core can return different codes which are not considered
a real error for this function.
Return success in such cases.

Signed-off-by: Eugen Hristev <eugen.hristev@collabora.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>