env: make env_import(_redund) return 0 on success, not 1

env: make env_import(_redund) return 0 on success, not 1

env_import (and env_import_redund) currently return 1 on success
and 0 on error. However, they are only used from functions
returning 0 on success or a negative value on error.

Let's clean this up by making env_import and env_import_redund
return 0 on success and -EIO on error (as was the case for all
users before).

Users that cared for the return value are also updated. Funny
enough, this only affects onenand.c and sf.c

Signed-off-by: Simon Goldschmidt <sgoldschmidt@de.pepperl-fuchs.com>
Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com>