cmd: ubifs: Move ubifs_initialized checking into cmd_ubifs_umount()

cmd: ubifs: Move ubifs_initialized checking into cmd_ubifs_umount()

cmd_ubifs_umount() function would be called directly instead of involving
whole command machinery in generic firmware loader, so checking on
ubifs_initialized status need to be done in cmd_ubifs_umount() without
breaking original functionality design.

Signed-off-by: Tien Fong Chee <tien.fong.chee@intel.com>
Reviewed-by: Marek Vasut <marex@denx.de>
Reviewed-by: Heiko Schocher <hs@denx.de>
[trini: Fix conflicting type error in cmd/ubi.c]
Signed-off-by: Tom Rini <trini@konsulko.com>