imgtool: Fix getpriv error return with private key

This commit fixes a bug with the getpriv command using
ECDSA keys.

Signed-off-by: Roland Mikhel <roland.mikhel@arm.com>
Change-Id: I66c1365a855e97199921ac136a18e26988bce508

scripts/imgtool/keys/ecdsa.py

@@ -179,7 +179,7 @@ def verify(self, signature, payload):
                         signature_algorithm=ec.ECDSA(SHA256()))
 
 
-class ECDSA256P1(ECDSA256P1Public, ECDSAPrivateKey):
+class ECDSA256P1(ECDSAPrivateKey, ECDSA256P1Public):
     """
     Wrapper around an ECDSA (p256) private key.
     """
@@ -252,7 +252,7 @@ def verify(self, signature, payload):
                         signature_algorithm=ec.ECDSA(SHA384()))
 
 
-class ECDSA384P1(ECDSA384P1Public, ECDSAPrivateKey):
+class ECDSA384P1(ECDSAPrivateKey, ECDSA384P1Public):
     """
     Wrapper around an ECDSA (p384) private key.
     """