Fix bad return value checks (detected with Coccinelle)

In the "Getting Started with Coccinelle - KVM edition" presentation that
has been held by Julia Lawall at the KVM forum 2015 (see the slides at
http://events.linuxfoundation.org/sites/events/files/slides/tutorial_kvm_0.pdf

),
she pointed out some bad return value checks in U-Boot that can be
detected with Coccinelle by using the following config file:

@@
identifier x,y;
identifier f;
statement S;
@@
x = f(...);
(
 if (x < 0) S
|
 if (
-     y
+     x
 < 0) S
)

This patch now fixes these issues.

Signed-off-by: Thomas Huth <huth@tuxfamily.org>