usb: pd: policy_engine: Handle spec revision properly

As per USB PD 3.0 specification the 2-bit Specification Revision field
of a GoodCRC Message does not carry any meaning and Shall be considered
as don't care by the recipient of the Message. Hence set spec revision
to USBPD_REV_20 for GoodCRC message. PD protocol message header defaults
to USBPD_REV_20 which gets updated USBPD_REV_30 if PD 3.0 source capabale
of programmable power supply.

Change-Id: I5209bb00c437f3c959d093a972f1ac75233cbcf9
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>