sf: winbond: fix page_size

Commit a4ed3b65 "sf: inline data constants" modified winbond.c's page_size
from 256 to 4096. This prevents either/both of "sf write" writing the
correct data, or "sf read" from reading the correct data back.

This allows U-Boot running on Compulab Tegra to upgrade itself.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>