Blackfin: nand: flush peripheral before polling it

We need to make sure the data written to the nand flash controller makes
it there before we start polling its status register.  Otherwise, we may
get stale data and return before the controller is actually ready.

Signed-off-by: Graf Yang <graf.yang@analog.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Acked-by: Scott Wood <scottwood@freescale.com>