sdhci: 'scratch' may be used uninitialized

The variable 'scratch' is always initialized before it's used. The
conditional which is responsible for initialization of 'scratch' will
always evaluate 'true' when the first loop iteration occurs, and thus,
it's properly initialized. GCC doesn't see this, of course, so using
the uninitialized_var() macro seems to work for silencing this case.

Signed-off-by: Steven Noonan <steven@uplinklabs.net>
Signed-off-by: Pierre Ossman <drzeus@drzeus.cx>