Fix bash completion for `plugin enable|disable`

`docker plugin enable` and `docker plugin disable` only
accept one plugin.

Signed-off-by: Harald Albers <github@albersweb.de>

contrib/completion/bash/docker

@@ -3222,7 +3222,10 @@ _docker_plugin_disable() {
 			COMPREPLY=( $( compgen -W "--help" -- "$cur" ) )
 			;;
 		*)
-			__docker_complete_plugins_installed
+			local counter=$(__docker_pos_first_nonflag)
+			if [ $cword -eq $counter ]; then
+				__docker_complete_plugins_installed
+			fi
 			;;
 	esac
 }
@@ -3239,7 +3242,10 @@ _docker_plugin_enable() {
 			COMPREPLY=( $( compgen -W "--help --timeout" -- "$cur" ) )
 			;;
 		*)
-			__docker_complete_plugins_installed
+			local counter=$(__docker_pos_first_nonflag '--timeout')
+			if [ $cword -eq $counter ]; then
+				__docker_complete_plugins_installed
+			fi
 			;;
 	esac
 }