usb: pd: Rerun usbpd_sm if RX queue is not empty

Upon receiving a PD message the state machine is queued to
act on it. However, if another message is received quickly
before usbpd_sm is scheduled, it will only process the first
message. Add a check if the RX queue is not empty in order
to drain the messages properly.

Change-Id: Ie9e079a51a5740024c9a1be5ba27b3edacf9b2df
Signed-off-by: Jack Pham <jackp@codeaurora.org>