ipmi: Cleanup DEBUG_TIMING ifdef usage

The driver uses #ifdef DEBUG_TIMING in order to conditionally print out
timestamped debug messages. Unfortunately it adds the ifdefs all over the
usage sites.

This patch cleans it up by adding a debug_timestamp() function which
is compiled out if DEBUG_TIMING isn't present. This cleans up all
the ugly ifdefs in the function logic.

Cc: openipmi-developer@lists.sourceforge.net
Cc: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: John Stultz <john.stultz@linaro.org>
Signed-off-by: Corey Minyard <minyard@mvista.com>