trace: Reduce the number of function sites

Given that the compiler adds two function calls into each function, the
current spacing is overkill. Drop it down to 16 bytes per function, which
is still plenty. This saves some space in the trace buffer.

Also move the calculation into a function, so it is common code. Add a
check for gd->mon_len being unset, which breaks tracing.

Signed-off-by: Simon Glass <sjg@chromium.org>