mvsdio: fix config failure with some high speed SDHC cards

Especially with Sandisk SDHC cards, the second SWITCH command was failing
with a timeout and the card was not recognized at all.  However if the
system was busy, or debugging was enabled, or a udelay(100) was inserted
before the second SWITCH command in the core code, then the timing was
so that the card started to work.

With some unusual block sizes, the data FIFO status doesn't indicate a
"empty" state right away when the data transfer is done.  Queuing
another data transfer in that condition results in a transfer timeout.

The empty FIFO bit eventually get set by itself in less than 50 usecs
when it is not set right away. So let's just poll for that bit before
configuring the controller with a new data transfer.

Signed-off-by: Nicolas Pitre <nico@marvell.com>
Signed-off-by: Pierre Ossman <pierre@ossman.eu>