pd: make VDO responses faster
pd: make VDO responses faster

Allow VDO responses to be sent faster by taking out the check for
incoming packet when a VDO is pending. This check isn't needed
because we already check if the PD state machine is busy sending
something.

With this change, the turn around time for responding to Discover
Identity on zinger is ~200us.

BUG=chrome-os-partner:35327
BRANCH=samus
TEST=loaded onto zinger and used twinkie to verify that discover
identity is responded to in ~200us every time. used ectool to
perform remote update on zinger, now takes ~18s (compared to ~55s).
Also, used following bash loop to constantly sent PD voltage requests:

while true; do dut-control "usbpd_uart_cmd:pd 1 dev 5"; sleep 0.3;
dut-control "usbpd_uart_cmd:pd 1 dev 20"; sleep 0.3; done

Used bash loop while updating zinger via ectool. I programmed zinger
~50 times and verified:
- we never missed a PD voltage request
- never got any PD protocol or phy layer errors (no collisions)
- zinger successfully jumped to RW after each (no packets missed)

Note: sending any other PD traffic while programming zinger does
obviously slow down zinger update (~30s with my bash loop above).

Change-Id: I94d1ac01440d096671972fa9c21c149ea432863f
Signed-off-by: Alec Berg <1a4429eeda29095a12e19a73405f80f5b7f37a63@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/240150
Reviewed-by: Todd Broch <035f5b1f817cbb94019cec357b99778115602b63@chromium.org>
Reviewed-by: Vincent Palatin <70a9964ec8fd10b0b08fcc8623c2add25ddf99ac@chromium.org>

coreboot/chrome-ec,coreboot/chrome-ec,fourier49/BZ_DEV_EC,md5555/ec,coreboot/chrome-ec,eatbyte/chromium-ec,akappy7/ChromeOS_EC_LED_Diagnostics,mtk09422/chromiumos-platform-ec,mtk09422/chromiumos-platform-ec,akappy7/ChromeOS_EC_LED_Diagnostics,coreboot/chrome-ec,eatbyte/chromium-ec,fourier49/BZ_DEV_EC,akappy7/ChromeOS_EC_LED_Diagnostics,fourier49/BIZ_EC,md5555/ec,fourier49/BIZ_EC,mtk09422/chromiumos-platform-ec,fourier49/BIZ_EC,akappy7/ChromeOS_EC_LED_Diagnostics,eatbyte/chromium-ec,mtk09422/chromiumos-platform-ec,akappy7/ChromeOS_EC_LED_Diagnostics,md5555/ec,md5555/ec,fourier49/BZ_DEV_EC,coreboot/chrome-ec,eatbyte/chromium-ec,coreboot/chrome-ec,fourier49/BIZ_EC,fourier49/BZ_DEV_EC