mmc: core: improve mmc_of_parse_voltage() to return better status

Improve mmc_of_parse_voltage()'s return values so that drivers can tell
whether a voltage-range specification was present, and whether it has
been successfully parsed, or there was an error while parsing.

We return a negative errno when parsing fails, zero if no voltage-range
specification is present, or one if a voltage-range specification is
successfully parsed.

No users need modifying as no users check the return value.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>