[MTD] [NAND] Verify probe by retrying to checking the results match

With modern systems using bus-hold instead of bus pull-up, it can
often lead to erroneous reporting of NAND devices where there are
none. Do a double probe to ensure that the result we got the first
time is repeatable, and if it is not then return that there is no
chip there.

Signed-off-by: Ben Dooks <ben-linux@fluff.org>
Signed-off-by: David Woodhouse <dwmw2@infradead.org>