add choices to words argument for recovery_device

trezorctl

@@ -349,6 +349,8 @@ class Commands(object):
     set_u2f_counter.help = 'Set U2F counter'
     firmware_update.help = 'Upload new firmware to device (must be in bootloader mode)'
 
+    clear_session.arguments= ()
+
     get_address.arguments = (
         (('-c', '--coin'), {'type': str, 'default': 'Bitcoin'}),
         (('-n', '-address'), {'type': str}),
@@ -400,7 +402,7 @@ class Commands(object):
     wipe_device.arguments = ()
 
     recovery_device.arguments = (
-        (('-w', '--words'), {'type': int}),
+        (('-w', '--words'), {'type': int, 'choices': [12, 18, 24], 'default': 24}),
         (('-p', '--pin-protection'), {'action': 'store_true', 'default': False}),
         (('-r', '--passphrase-protection'), {'action': 'store_true', 'default': False}),
         (('-l', '--label'), {'type': str, 'default': ''}),