usbkm232: Update readme for servo V4 routing

Servo V2/V3 have to explicitly route AVR to USB,
but it's hardwired on Servo V4. Update readme to reflect this.

BUG=None
TEST=None

Change-Id: I8152dc8d37040b1d65d2211fa52da4e553e2146a
Reviewed-on: https://chromium-review.googlesource.com/1173159
Commit-Ready: Ravi Chandra Sadineni <ravisadineni@chromium.org>
Tested-by: Nick Sanders <nsanders@chromium.org>
Reviewed-by: Ravi Chandra Sadineni <ravisadineni@chromium.org>
1 file changed