mmc: atmel-mci: not busy flag has also to be used for read operations

Even if the datasheet says that the not busy flag has to be used only
for write operations, it's false except for version lesser than v2xx.

Not waiting on the not busy flag for read operations can cause the
controller to hang-up during the initialization of some SD cards
with DMA after the first CMD6 -- the next command is sent too early.

Signed-off-by: Ludovic Desroches <ludovic.desroches@atmel.com>
Cc: stable <stable@vger.kernel.org> [3.5, 3.6]
Signed-off-by: Chris Ball <cjb@laptop.org>