[MTD] [OneNAND] Avoid deadlock in erase callback; release chip lock first.

When the erase callback performs some other action on the flash, it's
highly likely to deadlock unless we actually release the chip lock
before calling it.

This patch mirrors that same change already done for NAND.

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