usb: pd: Miscellaneous compliance fixes

Fix several issues which help address PD compliance testing:
- Soft reset should be issued instead of hard reset in cases
  where an unexpected message is received.
- In source mode, send a Reject if the Request PDO object also
  exceeds the advertised current capability. Fix the incorrect
  bitmask in the PD_RDO_FIXED_CURR macro.
- Handle possibility of quick back-to-back RX messages by
  flushing the sm worker to ensure the last message was fully
  processed before overwriting the buffer.
- Discard PING messages as they are not applicable to Type-C.
- Respond to GET_SINK_CAP while in source mode, and vice
  versa, respond to GET_SOURCE_CAP when in sink mode.
- Move pd->hard_reset=false to PE_SRC/SNK_TRANSITION_TO_DEFAULT
  for consistency.
- Change default sink capabilities to advertise only 5V @ 900mA
  since the policy engine will not automatically request higher
  voltages. If userspace wants to request greater than 5V it should
  also update the sink capabilities.
- Reset the protocol counters at the beginning of PE_SNK_STARTUP.

Change-Id: I44598eb0b49efd763f86f303c70e8b018fca994b
Signed-off-by: Jack Pham <jackp@codeaurora.org>