glados_pd: clean-up end of PD transmission
glados_pd: clean-up end of PD transmission

Clean up pd_tx_disable() so that it doesn't toggle a couple times
when we stop transmitting. Need to first set the CC pin to Hi-Z
and then the TX_DATA line since the CC pin is normally held low
during transmission.

BUG=none
BRANCH=none
TEST=test on glados. use twinkie to capture traffic and note that
without this change, the CC line toggles an extra time at the end
of each transmission. With this change it is a lot cleaner.

Change-Id: If26c7a10bbb08bc55b972bb0145115836579d37b
Signed-off-by: Alec Berg <1a4429eeda29095a12e19a73405f80f5b7f37a63@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/274884
Reviewed-by: Vincent Palatin <70a9964ec8fd10b0b08fcc8623c2add25ddf99ac@chromium.org>

akappy7/ChromeOS_EC_LED_Diagnostics,fourier49/BZ_DEV_EC,coreboot/chrome-ec,akappy7/ChromeOS_EC_LED_Diagnostics,coreboot/chrome-ec,md5555/ec,md5555/ec,akappy7/ChromeOS_EC_LED_Diagnostics,fourier49/BZ_DEV_EC,fourier49/BIZ_EC,coreboot/chrome-ec,akappy7/ChromeOS_EC_LED_Diagnostics,coreboot/chrome-ec,akappy7/ChromeOS_EC_LED_Diagnostics,md5555/ec,fourier49/BIZ_EC,coreboot/chrome-ec,fourier49/BZ_DEV_EC,fourier49/BIZ_EC,md5555/ec,fourier49/BZ_DEV_EC,fourier49/BIZ_EC,coreboot/chrome-ec