vboot: fix invalid check for the returned value from spi_flash->write

vboot: fix invalid check for the returned value from spi_flash->write

spi_flash->write returns non-zero on error and zero on success, not the
number of bytes written.

BUG=none
BRANCH=ToT
TEST=Booted storm. Verified successfully nvdata was saved.

Original-Change-Id: If50cc1a62a4f06398d1830cca60085b6f925fff3
Original-Signed-off-by: Daisuke Nojiri <dnojiri@chromium.org>
Original-Reviewed-on: https://chromium-review.googlesource.com/229389
Original-Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Original-Reviewed-by: Randall Spangler <rspangler@chromium.org>
(cherry picked from commit 1e8cdbdb07e99c3f72c35f76d68144f46107acd9)
Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Change-Id: I216e97f734da8d4b52c2da8329f4143b7b0656cd
Reviewed-on: http://review.coreboot.org/9439
Reviewed-by: Patrick Georgi <pgeorgi@google.com>
Tested-by: build bot (Jenkins)