mtd: spi-nor: Support controllers with limited TX FIFO size

Some SPI controllers can't write nor->page_size bytes in a single
step because their TX FIFO is too small.

Allow nor->write() to return a size that is smaller than the requested
write size to gracefully handle this case.

Signed-off-by: Yogesh Gaur <yogeshnarayan.gaur@nxp.com>
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>