libertas: Remove function entry/exit debugging

In at least one place, the enter/exit debugging was not being correctly
matched. Based on mailing list feedback, it was desired to drop all of
these in favor of using ftrace instead.

Suggested-by: Joe Perches <joe@perches.com>
Suggested-by: Kalle Valo <kvalo@codeaurora.org>
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>