msm8960-common: libril: support v6 signal strength as well

* Recent HTC radios are using what appears to be a standard v6 signal
  strength format again.

Change-Id: Iea3bd09b43a47d57daccfe5d1c5f6fa9e024ea99
Merge pull request #1 from CyanogenMod/cm-10.2

msm8960-common: libril: support v6 signal strength as well