ec_uartd: bugfix for buffer pointer on partial write to FD.
ec_uartd: bugfix for buffer pointer on partial write to FD.

Previously I neglected to increment the buffer pointer which would
allow double writing of same data and exclude tail if write didn't
complete in one call.

Also fixing bug with EAGAIN/EWOULDBLOCK to retry in those cases.
Previously we'd just have just failed for those errno values.

Signed-off-by: Todd Broch <035f5b1f817cbb94019cec357b99778115602b63@chromium.org>

BRANCH=none
BUG=chromium:371147
TEST=still compiles

Change-Id: I30dbe56b2a4c735c487464349786a9db430130a8
Reviewed-on: https://chromium-review.googlesource.com/199052
Reviewed-by: Vincent Palatin <70a9964ec8fd10b0b08fcc8623c2add25ddf99ac@chromium.org>
Commit-Queue: Todd Broch <035f5b1f817cbb94019cec357b99778115602b63@chromium.org>
Tested-by: Todd Broch <035f5b1f817cbb94019cec357b99778115602b63@chromium.org>

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