spl: spl_mmc: fix mmc Falcon mode regression

91199f4a broke mmc based Falcon mode.

The block_read function returns the number of blocks read thus the error
check needs to look for a return of 0 blocks read.

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