[MTD] mtdoops: Various minor cleanups

Various minor cleaups to mtdoops:
  * Don't support the mtd->erasesize < OOPS_PAGE_SIZE case
  * Tweak printks and make the device mtdoops connects to more visible
  * CON_PRINTBUFFER flag is uneeded

Signed-off-by: Richard Purdie <rpurdie@rpsys.net>
Signed-off-by: David Woodhouse <dwmw2@infradead.org>