mtd: nand: Wait for PAGEPROG to finish in drivers setting NAND_ECC_CUSTOM_PAGE_ACCESS

Drivers setting NAND_ECC_CUSTOM_PAGE_ACCESS are supposed to handle the
full read/write page sequence, and waiting for a page to actually be
programmed is part of this write-page sequence.
This is also what is done in ->write_oob_xxx() hooks, so let's do that in
->write_page_xxx() as well to make it consistent.

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>