spl_mmc.c: Detect missing kernel image in RAW MMC

Currently, we assume that if we can read from MMC correctly, we have
found a valid image.  This is not the case as an empty area will read
just fine.  Add a check for a valid IH_MAGIC.

Signed-off-by: Tom Rini <trini@ti.com>
Reviewed-by: Peter Korsgaard <jacmet@sunsite.dk>