stm32 spi: postpone the RX DMA setup if handler is still using buffer.
stm32 spi: postpone the RX DMA setup if handler is still using buffer.

(cherry-pick back to ToT)

If the AP de-asserts the SPI NSS pin while host command handler is
still processing the command, we would delay the Rx DMA setup later.
If this case happens, the pending result of handler will be dropped.

BUG=chrome-os-partner:28979
BRANCH=tot,nyan
TEST=build and play around on big.

Signed-off-by: Louis Yung-Chieh Lo <2d80bee8408e64f859a1b5868bd3b311c0a4b4cd@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/204427
Reviewed-by: Randall Spangler <62698fdbb84d1779579ee80c3f39fac22017e5bc@chromium.org>

(cherry picked from commit 4be73492817f3f6c24ece75fed33eb956c0038b8)

Change-Id: Ie2a6550696760eadad3b0d6e3a4e56a2b29abdda
Original-Change-Id: I371a2a0b96b1ee0602be91338bd53d13f6abbd2e
Reviewed-on: https://chromium-review.googlesource.com/206922
Reviewed-by: Randall Spangler <62698fdbb84d1779579ee80c3f39fac22017e5bc@chromium.org>
Commit-Queue: Yung-chieh Lo <2d80bee8408e64f859a1b5868bd3b311c0a4b4cd@chromium.org>
Tested-by: Yung-chieh Lo <2d80bee8408e64f859a1b5868bd3b311c0a4b4cd@chromium.org>

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