fb_mmc.c: Correct blk_dread() return value checks

The function blk_dread will return -ENOSYS on failure or on success the
number of blocks read, which must be the number asked to read (otherwise
it failed somewhere).  Correct this check.

Cc: Lukasz Majewski <lukma@denx.de>
Signed-off-by: Tom Rini <trini@konsulko.com>