imx: spl_imx_romapi: fix emmc fast boot mode case

This fixes a regression in the eMMC fast boot mode case where the buffer
was missing 464 bytes.

The code figures out how many bytes must at least be fetched to honor
the current read, rounds that up to the ss->pagesize [which is a no-op
in the USB download case because that has ->pagesize==1], fetches that
many bytes, but then recorded the original upper bound as the new end of
the valid data. However, this did not take into account the rounding up
to the ss->pagesize. Fix this by recording the actual bytes downloaded.

Fixes: 4b447243 ("imx: spl_imx_romapi: avoid tricky use of spl_load_simple_fit() to get full FIT size")
Signed-off-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Acked-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>
Reviewed-by: Fabio Estevam <festevam@gmail.com>