jffs2: update to new MTD interface

There were a few instances of the old MTD interface remaining for JFFS2. We
fix one error that shows up (only when CONFIG_JFFS2_FS_WRITEBUFFER is not
defined) like this:

  fs/jffs2/read.c: In function 'jffs2_read_dnode':
  fs/jffs2/read.c:36:8: error: 'struct mtd_info' has no member named 'read'
  fs/jffs2/read.c:112:8: error: 'struct mtd_info' has no member named 'read'
  ...

We also simply remove two macros that are not in use, were not updated to
the new MTD interface, and don't even utilize the old interface properly.
(That means they weren't used since commit 8593fbc6, year 2006; almost 6
years ago, for those who don't want to do the math)

Reported-by: Randy Dunlap <rdunlap@xenotime.net>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Acked-by: Randy Dunlap <rdunlap@xenotime.net>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>