[MTD] OneNAND: Invalidate bufferRAM after erase

OneNAND has internal bufferRAMs.  The driver keeps track of
what is in the bufferRAM to save having to load from the
NAND core.  After an erase operation, the driver must
mark bufferRAM invalid if it refers to the erased block.

Signed-off-by: Adrian Hunter <ext-adrian.hunter@nokia.com>
Signed-off-by: David Woodhouse <dwmw2@infradead.org>