mtd: nand: pxa3xx: Use a completion to signal device ready

The expected behavior of the waitfunc() NAND chip call is to wait
for the device to be READY (this is a standard chip line).
However, the current implementation does almost nothing, which opens
the possibility of issuing a command to a non-ready device.

Fix this by adding a new completion to wait for the ready event to arrive.

Signed-off-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Tested-by: Daniel Mack <zonque@gmail.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>