wdctl: remove -d option leftover

wdctl: remove -d option leftover

-d option was removed in commit f56338b43973 ("wdctl: allow to specify more
than one device"), but the optstring wasn't updated at that time and wdctl
can still accept the option halfway as below:

 $ wdctl -d
 wdctl: option requires an argument -- 'd'

whereas it should say:

 wdctl: invalid option -- 'd'

So update the optstring.

Fixes: f56338b43973 ("wdctl: allow to specify more than one device")
Signed-off-by: Munehisa Kamata <kamatam@amazon.com>
(cherry picked from commit 56e2c86c2c7ea012b63cd896d9ca3daa34f19565)