mtd: nand: invalidate cache on unaligned reads

In rare cases, we are given an unaligned parameter `from' in
`nand_do_read_ops()'. In such cases, we use the page cache
(chip->buffers->databuf) as an intermediate buffer before dumping to the
client buffer. However, there are also cases where this buffer is not
cleanly reusable. In those cases, we need to make sure that we
explicitly invalidate the cache.

This patch prevents accidental reusage of the page cache, and for me,
this solves some problems I come across when reading a corrupted BBT
from flash (NAND_BBT_USE_FLASH and NAND_BBT_NO_OOB).

Note: the rare "unaligned" case is a result of the extra BBT pattern +
version located in the data area instead of OOB.

Also, this patch disables caching on raw reads, since we are reading
without error correction. This is, obviously, prone to errors and should
not be cached.

Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@intel.com>