sunxi: mmc: A20: Fix MMC optimisation

Some SoCs (as seen on A20) seem to misreport the MMC FIFO level if the
FIFO is completely full: the level size reads as zero, but the FIFO_FULL
bit is set. We won't do a single iteration of the read loop in this
case, so will be stuck forever.

Check for this situation and use a safe minimal FIFO size instead when
we hit this case.

This fixes MMC boot on A20 devices after the MMC FIFO optimisation
(9faae545).

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Jaehoon Chung <jh80.chung@samsung.com>