ubifs: Fix bad free() sequence in ubifs_finddir()

Free private_data member element before freeing file structure.
This was causing malloc to crash. Also remove unnecessary variable
assigments as file structure gets free'd as well.

Signed-off-by: Rod Boyce <uboot@teamboyce.co.uk>
Signed-off-by: Wolfgang Denk <wd@denx.de>
Cc: Stefan Roese <sr@denx.de>
Signed-off-by: Stefan Roese <sr@denx.de>