[MTD] Correct partition failed erase address

If an erase operation fails, the address at which the
failure occurred is returned by the driver.  The MTD
partition must adjust this address (by subtracting the
partition offset) before returning to the caller.
This was not happening, which caused JFFS2 to mark
the wrong block bad!

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