trezor: single passphrase entry

Only require the user to input the passphrase once, unless creating
a wallet.
Should they mis-enter the passphrase, they will be warned Electrum
couldn't pair the device, and when they actually need to use it
they will be prompted again.
Fixes #1672

plugins/keepkey/cmdline.py

@@ -3,7 +3,7 @@ from electrum.util import print_msg
 
 class KeepKeyCmdLineHandler:
 
-    def get_passphrase(self, msg):
+    def get_passphrase(self, msg, confirm):
         import getpass
         print_msg(msg)
         return getpass.getpass('')