hdctools: servod: bugfix to retry failed i2c slave acks

Due to issues with timely transactions across usb->i2c->usb its
necessary to retry transactions.  Retry logic had been added for all
failures accept slave acks.  This CL fixes that bug noting that we
must limit the number of consecutive retries to a slave as it may be a
legitimate nack ( non-existent or non-responsive slave ).

BUG=chrome-os-partner:4717
TEST=manual,

Run,
sudo servod -c servo.xml -c servo_loc.xml &
dut-control i2c_mux:loc0

while true ; do
  dut-control i2c_mux_en:on loc_0x40_cfg_reg i2c_mux_en:off loc_0x40_cfg_reg
  sleep 1
done

Check,

dut-control output:
  loc_0x40_cfg_reg:0x399f
  2011-06-28 17:02:58,468 - root - ERROR ...
  <repeat>

servod output:
  <make sure ack retries occur 10 times then fail>

Change-Id: If5bd30ead2ea740a4039a5393717710a2cab2c81
Reviewed-on: http://gerrit.chromium.org/gerrit/3344
Reviewed-by: Mark Hayter <mdhayter@chromium.org>
Tested-by: Todd Broch <tbroch@chromium.org>
2 files changed