Fix "ubi part" cmd re-entrancy

Commit 2ee951ba (UBI: Enable re-initializing of the "ubi part" command)
reset mtd_devs in ubi_exit() but missed ubi_init()'s failure path.

Signed-off-by: Karl Beldan <karl.beldan@gmail.com>
Cc: Stefan Roese <sr@denx.de>
Signed-off-by: Stefan Roese <sr@denx.de>