mmc: mtk-sd: fix possible incomplete read ops

mmc: mtk-sd: fix possible incomplete read ops

The code is checking for incomplete read when it see the INT_XFER_COMPL
flag, but it forget to first check whether there is anything left in the
FIFO to copy to the RX buffer. This means that sometimes we will get
errors because of erroneous incomplete read operation.

This commit fixes the driver re-ordering the code so that we first
check for data inside the RX fifo and only after check the status
of the INT_XFER_COMPL flag.

Signed-off-by: Fabien Parent <fparent@baylibre.com>