mtd_dataflash: unbreak erase support

Commit 5b7f3a50

 (fix dataflash 64-bit divisions) unfortunately
introduced a typo. Erase addr and len were swapped in the pageaddr
calculation, causing the wrong sectors to get erased.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>
Acked-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>