[MMC] Change SDHCI version error to a warning

O2 Micro's controllers have a larger specification version value and are
therefore denied by the driver. When bypassing this check they seem to work
fine. This patch makes the code a bit more forgiving by changing the error
to a warning.

Signed-off-by: Pierre Ossman <drzeus@drzeus.cx>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>