We know how many bytes the EC should be sending us (which is also the
number of bytes transferred) and also how many bytes the EC actually
wanted to send to us.  When computing the checksum and copying back
data let's make sure we take the lesser of the two of those.  We'll
also complain if the EC tried to send us too many bytes.  The EC
sending us too few bytes is legit for when we send the EC an invalid
command.

This is based on similar code in cros_ec_spi.
Signed-off-by: Doug Anderson <dianders@chromium.org>
Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>