usb: pd: Abort TX upon late RX

In general receiving a PD message should preempt a message
transmission in progress or about to be sent. Improve this
behavior by moving the check for any newly received messages
into pd_send_msg(). Also within the PD PHY driver, try to catch
scenarios in which an RX message arrives after pd_phy_write()
has been called but just before the TX register write operation
by checking the rx_msg_cnt counter to see if it changed since
the start of the function call.

Change-Id: I4084d18940b36df1f6eab8b0256ef30a81369884
Signed-off-by: Jack Pham <jackp@codeaurora.org>