fixes MCPhaseGate definition (#4989) (#4995)

Co-authored-by: Kevin Krsulich <kevin.krsulich@ibm.com>
(cherry picked from commit 08a1f37)

Co-authored-by: Faisal Debouni <faisal@megoapps.com>
Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>

qiskit/circuit/library/standard_gates/p.py

@@ -260,9 +260,9 @@ def _define(self):
         else:
             from .u3 import _gray_code_chain
             scaled_lam = self.params[0] / (2 ** (self.num_ctrl_qubits - 1))
-            bottom_gate = PhaseGate(scaled_lam)
+            bottom_gate = CPhaseGate(scaled_lam)
             definition = _gray_code_chain(q, self.num_ctrl_qubits, bottom_gate)
-            qc.data = definition
+            qc._data = definition
         self.definition = qc
 
     def control(self, num_ctrl_qubits=1, label=None, ctrl_state=None):