cmd: ubifs: Factor out some checking codes into cmd_ubifs_mount()

cmd: ubifs: Factor out some checking codes into cmd_ubifs_mount()

cmd_ubifs_mount() function would be called directly instead of
involving whole command machinery for mounting ubifs in
generic firmware loader, so some checking codes need to be factored out
into cmd_ubifs_mount() 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>