dmctl: Do not skip argument if not matched with '-ro'.

'dmctl create' command would fail if '-ro' flag is not
used. This happend because the argument in place of '-ro'
is skipped unconditionally.

Bug: 72722987
Test:
 $ losetup /dev/block/loop0 fs.img
 $ dmctl create FOO linear 0 25600 /dev/block/loop0 0

Change-Id: I3f6264336b9c9a1b5de76f3a1d29e6045453f9cb
Signed-off-by: Sandeep Patil <sspatil@google.com>