usb: pd: pdphy: Acknowledge Rx sooner

To avoid missing an incoming packet that was quickly sent while
still processing the previous packet, clear the acknowledge bit
as soon as the message is consumed from the Rx FIFO. This gives
the hardware a chance to send a GoodCRC without causing the
sender to retry.

Change-Id: I8a9ef42685576ab68aa65efab1bebc40d801990d
Signed-off-by: Jack Pham <jackp@codeaurora.org>