[U-Boot] spl: spl_mmc: Error and count distinction

[Paul Kocialkowski]

My previous commit:
spl: spl_mmc: Clearer structure in spl_mmc_load_image and cosmetics

introduced a regression due to a wrongly-checked return code, that represented
a read bytes count and not an error. This was fixed by commit:
spl: spl_mmc: fix mmc Falcon mode regression

This commit aims to clarify whether each return code is a read bytes count or
an error in order to avoid such mistakes in the future. It applies on top of:
spl: spl_mmc: fix mmc Falcon mode regression