mmc: core: correct invalid error checking

The effect of the existing code is that we continue blindly when we
should warn about an invalid allocation unit.

Reported-by:  <dcb314@hotmail.com>
Resolves-bug: https://bugzilla.kernel.org/show_bug.cgi?id=44061


Signed-off-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Chris Ball <cjb@laptop.org>