util/comm-i2c: return response length from `ec_command`
util/comm-i2c: return response length from `ec_command`

In my original implementation I returned the response code (possibly
because I was confused by `ec_command_servo_spi` returning something
like that), but `do_i2c_xfer` (used for all I2C passthrough stuff)
breaks in that case. Changing it to return response length (consistent
with the `ec_command` doc comment) fixes that problem.

BRANCH=none
BUG=chromium:1008568
TEST=Use `ectool i2cwrite ...` to write to one of the EC's I2C slaves
     (should report success instead of "Truncated read response").

Change-Id: I341c3907ac44c6c3d1086ae10b31e81062215f20
Signed-off-by: Harry Cutts <1d9acf085666736c25fb5139ba37ad9d917ae7f5@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/1830099
Reviewed-by: Daisuke Nojiri <fd5f93af191bf7e8f73ea71cc3c0f66b41b1dd49@chromium.org>

coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec