According to the Linux kernel and the qemu code, the MMCIFIFOCNT contains
the remaining number of words to read, excluding those that are already in
the FIFO. So the current number of bytes in the FIFO is
host_remain - (readl(base + MMCIFIFOCNT) << 2).

Also writing to MMCIDATACTRL will reset the read counter. As a result,
MCI_DATABLOCKEND is never set and read_bytes() waits forever.

With this change, SD-Card support on qemu vexpress works correctly.

Signed-off-by: Michael Olbrich <m.olbrich@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>