mmc: if possible, poll the busy state using DAT0

mmc: if possible, poll the busy state using DAT0

Using the DAT0 line as a rdy/busy line is an alternative to reading the
status register of the card. It especially useful in situation where the
bus is not in a good shape, like when modes are switched.
This is also how the linux driver behaves.

Note of warning: As per the specification, while polling on DAT0 the CLK
must not turned off: "[...] Without a clock edge the Device (unless
previously disconnected by a deselect command (CMD7)) will force the DAT0
line down, forever. [...]"

Signed-off-by: Jean-Jacques Hiblot <jjhiblot@ti.com>