spl: spl_load: fix comparison between negative error code and unsigned size

read could be a negative error value but size in spl_image is unsigned
so when they are compared read is used as if it's a unsigned value
and if it's negative it'll most likely be bigger than size and the
result will be true and _spl_load() will return 0 to the caller.

This results in the caller to _spl_load() not seeing that an error happened
as it should and continuing as if the load was completed when it might
not have been.

Check if read is negative and return it's value if it is before comparing
against size in spl_image.

Signed-off-by: Daniel Palmer <daniel@0x0f.com>
Reviewed-by: Simon Glass <sjg@chromium.org>