[MTD] [OneNAND] Allow for controller errors when reading

A power loss while writing can result in a page becoming unreadable.  
When the device is mounted again, reading that page gives controller 
errors. Upper level software like JFFS2 treat -EIO as fatal, refusing to 
mount at all.  That means it is necessary to treat the error as an ECC 
error to allow recovery. Note that typically in this case, the 
eraseblock can still be erased and rewritten i.e. it has not become a 
bad block.

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