regulator: anatop: allow regulator to be in bypass mode

Bypass support was added in commit d38018f2 ("regulator: anatop: Add
bypass support to digital LDOs"). A check for valid voltage selectors was
added in commit da0607c8 ("regulator: anatop: Fail on invalid voltage
selector") but it also discards all regulators that are in bypass mode. Add
check for the bypass setting. Errors below were seen on a Variscite mx6
board.

anatop_regulator 20c8000.anatop:regulator-vddcore@140: Failed to read a valid default voltage selector.
anatop_regulator: probe of 20c8000.anatop:regulator-vddcore@140 failed with error -22
anatop_regulator 20c8000.anatop:regulator-vddsoc@140: Failed to read a valid default voltage selector.
anatop_regulator: probe of 20c8000.anatop:regulator-vddsoc@140 failed with error -22

Fixes: da0607c8 ("regulator: anatop: Fail on invalid voltage selector")
Signed-off-by: Mika Båtsman <mbatsman@mvista.com>
Signed-off-by: Mark Brown <broonie@kernel.org>