usb: pd: Avoid spurious state machine work on CC change

If an unexpected or delayed Type-C mode change notification
arrives from the charger driver, it could falsely kick the
state machine and could cause it to prematurely hard reset
since it preempted an expected message. This is particularly
true for the PR Swap operation in which the timing of PS_RDY
and resulting power negotiation is disrupted by the CC change
notification which is catching up. The previous way of handling
this by using the pd->in_pr_swap flag to mask the typec_mode
changes is quite fragile due to the uncertainty of when said
flag should be cleared.

Hence, demote the use of the in_pr_swap flag for only ignoring
the expected disconnect the happens in between the role change.
Then, for typec_mode changes, determine if current_pr is changed
or not to decide whether to kick the state machine. Finally,
since disconnect now is signaled by pd->current_pr==PR_NONE,
we'll need to keep track of whether VBUS had been enabled with
a separate flag rather than relying on pd->current_pr==PR_SRC.

Change-Id: I66be40091b75726d25c7be4478f1248a6af9ca3c
Signed-off-by: Jack Pham <jackp@codeaurora.org>