mtd: mtdoops: do not use mtd->panic_write directly

Instead of checking if 'mtd->panic_write' is defined, call 'mtd_panic_write()'
and check the error code - '-EOPNOTSUPP' will be returned if the function is
not defined.

Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>