mtd: cfi_probe: use P_ID_* definitions instead of hardcoded values

Use P_ID_* definitions already in include/linux/mtd/cfi.h instead of the
hardcoded values. Make the code more readable.

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>