ext4fs: le32_to_cpu() used on a 16-bit field

Fix reading ext4_extent_header struture on BE machines.  Some 16 bit
fields where converted to 32 bit fields, due to the byte swap on BE
machines the containing value was corrupted. Therefore reading ext4
filesystems on BE machines where broken before.

Signed-off-by: Rommel Custodio <sessyargc+uboot@gmail.com>
[sent via git-send-email; rework commit message]
Signed-off-by: Andreas Bießmann <andreas.devel@googlemail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Tested-by: Simon Glass <sjg@chromium.org>
Tested-by: Lukasz Majewski <l.majewski@samsung.com>