usb: pd: avoid missed connection notification

In the disconnect handler of usbpd_sm(), pd->current_pr is
already set to PR_NONE, so there is no reason to set it again.
This helps for cases where a quick reconnection notification
just arrived and sets the variable to either PR_SRC or PR_SINK
but the disconnect handling clears it resulting in nothing to
be done the next time work is scheduled.

Change-Id: Ia88520149905989a3b3efb94361670760b79c0bd
Signed-off-by: Jack Pham <jackp@codeaurora.org>