uboot-imx

spl: spl_mmc: Error and count distinction

This introduces a distinction between return codes that are read bytes counts
and errors. Read bytes counts are erroneous when null (no data was read) while
errors are erroneous when non-null.

Signed-off-by: Paul Kocialkowski <contact@paulk.fr>
Tested-by: Tim Harvey <tharvey@gateworks.com>