devfreq_power: return a DataFrame in get_all_freqs, like cpu_power do

Devfreq{In,Out}Power.get_all_freqs() should behave like
Cpu{In,Out}Power.  Make it return a DataFrame instead of a Series.

Apart from making it consistent, in pandas < 0.14, concatenating
DataFrame and Series raises a TypeError.  This fixes it.

Change-Id: Ib33408ff9dd475134a01813c822a483303df24a3
diff --git a/cr2/devfreq_power.py b/cr2/devfreq_power.py
index e09bbd6..23449cf 100644
--- a/cr2/devfreq_power.py
+++ b/cr2/devfreq_power.py
@@ -16,6 +16,8 @@
 """Process the output of the devfreq_cooling devices in the current
 directory's trace.dat"""
 
+import pandas as pd
+
 from cr2.base import Base
 from cr2.run import Run
 
@@ -32,14 +34,14 @@
         )
 
     def get_all_freqs(self):
-        """Return a pandas.Series with the frequencies for the devfreq device
+        """Return a pandas.DataFrame with the frequencies for the devfreq device
 
         The format should be the same as the one for
         CpuInPower().get_all_freqs().  Frequencies are in MHz.
 
         """
 
-        return self.data_frame["freq"] / 1000000
+        return pd.DataFrame(self.data_frame["freq"] / 1000000)
 
 Run.register_class(DevfreqInPower, "thermal")
 
@@ -56,7 +58,7 @@
         )
 
     def get_all_freqs(self):
-        """Return a pandas.Series with the output frequencies for the devfreq
+        """Return a pandas.DataFrame with the output frequencies for the devfreq
 device
 
         The format should be the same that that of
@@ -64,6 +66,6 @@
 
         """
 
-        return self.data_frame["freq"] / 1000000
+        return pd.DataFrame(self.data_frame["freq"] / 1000000)
 
 Run.register_class(DevfreqOutPower, "thermal")