drivers/spi_nor: fix writes across page boundaries

The previous code limited the length of a write to the size of a page,
but did not check whether the starting position was far enough into the
page that the write would still cross a page boundary.

NOTE: SPI NOR page size is fixed as 256 regardless of what Kconfig says.

Backport: 2.0 660149b5
Backport: 2.0 86dcf3c1

Signed-off-by: Peter A. Bigot <pab@pabigot.com>