spl: fix ext4fs_mount return code handling

- Despite other ext4 filesystem functions, ext4fs_mount returns
  0 in case of error.
- This leads to u-boot crash in case that an SD card
  with valid partition table but without ext4 filesystem created
  in a partition is found on SD card.
- Fix this by returning a proper error code of '-1' from spl_load_image_ext
  function in case of ext4fs_mount error.

Signed-off-by: Thomas Schaefer <thomas.schaefer@kontron.com>
[hthiery: slightly reword the commit message]
Signed-off-by: Heiko Thiery <heiko.thiery@gmail.com>
Reviewed-by: Tom Rini <trini@konsulko.com>