mtd: cfi_probe: make the addresses used to enter Auto Select Mode variable

Make the addresses used to enter Auto Select Mode variable to leave place
for handling chips using non-standard addresses.

Signed-off-by: Guillaume LECERF <glecerf@gmail.com>
Reviewed-by: Wolfram Sang <w.sang@pengutronix.de>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>