fs/squashfs: Set ret to 0 on successful read

fs/squashfs: Set ret to 0 on successful read

It might still be a positive number due to the call to sqfs_disk_read.
This only happens when reading a file from an uncompressed squashfs.

I found this by trying to boot using the extlinux bootmethod, where
positive values are treated as errors.

Signed-off-by: Michael Zimmermann <sigmaepsilon92@gmail.com>
Acked-by: Richard Genoud <richard.genoud@bootlin.com>
Reviewed-by: Simon Glass <sjg@chromium.org>