mtd: m25p80: Reinstate error print on unrecognized flash

Commit b34bc037 removed the
"unrecognized JEDEC id" error message, causing the probe function to
silently abort if the flash ID is unrecognized.

It is desirable to produce diagnostic output in this situation so that
the user has some idea what went wrong.

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>