usb: pd: Add string definitions for message types

Introduce string constants to correspond to the data, control
and extended message types. Use these to clean up all instances
of pd_send_msg() error logs, as they can now be consolidated
using a common format string. phy_msg_received() can get the
same treatment by logging the message type as a string instead
of a numeral which eases debugging.

Change-Id: I14ebc572b4cb164418d3856efddab1ec01c02eff
Signed-off-by: Jack Pham <jackp@codeaurora.org>