usb: pd: pdphy: Handle BIST messages

The QPNP PD PHY supports BIST modes as required by the USB PD
specification. Specifically for BMC signaling, upon reception
of BIST requests to enter Carrier Mode 2 or Test Data Mode,
place the PHY in the appropriate test mode when receiving
these messages in the RX handler.

While at it, remove the unnecessary kmalloc in the RX handler
as the temporary 32-byte buffer can be placed on the stack.

Change-Id: I79f406f7ecc9b938868c383743668df4e579a907
Signed-off-by: Jack Pham <jackp@codeaurora.org>