usb: pd: qpnp-pdphy: Remove rx_busy flag

Having this flag was causing PD TX messages to incorrectly
get blocked in the case where the policy engine state machine
worker got scheduled to run in the RX IRQ, but before the
handler exits and clears the flag. This caused false errors
that an RX message is pending. Remove usage of this flag as
it was doing more harm than good.

Change-Id: Iffccfc9ffafac6017192d86d8d97d5534890c7b5
Signed-off-by: Jack Pham <jackp@codeaurora.org>