[MMC] sdhci: check SDHCI base clock

A base clock value of 0 means that the driver must get the base clock through
some other means.  As we have no other way of getting it, we must abort.

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