sunxi: mmc: increase status register polling rate for data transfers

With a recent bunch of SD3.0 cards in our A20-based board we
experienced data transfer rates of about 250 KiB/s instead of 10 MiB/s
with previous cards from the same vendor (both 4 GB/class 10). By
increasing status register polling rate from 1 kHz to 1 MHz we were
able to reach the original transfer rates again. With the old cards
we now even reach about 16 MiB/s.

Signed-off-by: Tobias Doerffel <tobias.doerffel@ed-chemnitz.de>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>