regulator: of: Don't flag voltage change as possible for exact voltages

Flagging voltage changes as possible for exactly specified voltages
appears to be triggering bugs in the SDHCI code (it should be able to
handle the case where only one voltage it wants is in the range it is
allowed to set) so make sure we only set the flag in cases where there's
genuine variability.

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