mmc: sunxi: change controller error info to debug level

The controller's errors are usually normal (for example, for MMC or SDIO
cards, some errors are expected to happen; and for boards without a
dedicated card detect pin the error info will even flood console and
hide other normal messages) and hard to understand.

Change their print level to debug, thus it won't be shown to generic
users.

Signed-off-by: Icenowy Zheng <icenowy@aosc.xyz>
Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>