mtd: m25p80: Clean up JEDEC manufacturer checks

Use the manufacturer ID names from cfi.h instead of hard-coding
hex constants.  Introduce a JEDEC_MFR macro for clarity.

Signed-off-by: Kevin Cernekee <cernekee@gmail.com>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>