touchpad_st: fix heatmap mode
touchpad_st: fix heatmap mode

The rx_buf was set incorrectly in heat map mode.
Also add debug messages to help debugging.

BRANCH=none
BUG=b:70482333
TEST=test on full system, enable heatmap
Signed-off-by: Wei-Han Chen <5413a119f383c3f4549c3ef659403f393bf06e9d@chromium.org>

Change-Id: I325a9b7983a16634b95eecdc592c59222e48c7aa
Reviewed-on: https://chromium-review.googlesource.com/1177093
Commit-Ready: Wei-Han Chen <5413a119f383c3f4549c3ef659403f393bf06e9d@chromium.org>
Tested-by: Wei-Han Chen <5413a119f383c3f4549c3ef659403f393bf06e9d@chromium.org>
Reviewed-by: Nicolas Boichat <f3daba1cdc403799935674d3cd4b0e5bd1804232@chromium.org>

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