mmc: sunxi: drop superfluous error message

This error message can go because a) currently nothing else than
EPROBE_DEFER is returned and b) if this is going to change a much more
detailed error message should come from mmc_regulator_get_supply()
anyhow.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>