spl: spl_mmc: Disambiguate error message

The error message "spl: mmc block read error" may come from two
different functions, so we should better annotate the function name
where the error comes from to help debugging.

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Reviewed-by: Tom Rini <trini@konsulko.com>