mtd: OneNAND: move erase method to a separate function

Separate the actual execution of erase to a new function:
onenand_block_by_block_erase(). This is done in preparation for
the multiblock erase support.

Signed-off-by: Mika Korhonen <ext-mika.2.korhonen@nokia.com>
Reviewed-by: Adrian Hunter <adrian.hunter@nokia.com>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>