ipmi: Update timespec usage to timespec64

As part of the internal y2038 cleanup, this patch removes
timespec usage in the ipmi driver, replacing it timespec64

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>