mtd: nand: Prevent dereference of NULL pointer

mtd: nand: Prevent dereference of NULL pointer

In nand_wait_ready there is a loop that includes a NULL check for
chip->dev_ready before it is dereferenced. Use a NULL check once the
loop is exited as well to cover the case where it exits due to a timeout
and it is therefore not known if chip->dev_ready is NULL or not.

This issue found by Smatch.

Signed-off-by: Andrew Goodbody <andrew.goodbody@linaro.org>
Reviewed-by: Michael Trimarchi <michael@amarulasolutions.com>