ipmi: kill off 'timespec' usage again

'struct timespec' is getting removed from the kernel. The usage in ipmi
was fixed before in commit 48862ea2 ("ipmi: Update timespec usage
to timespec64"), but unfortunately it crept back in.

The busy looping code can better use ktime_t anyway, so use that
there to simplify the implementation.

Fixes: cbb19cb1 ("ipmi_si: Convert timespec64 to timespec")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Message-Id: <20191108203435.112759-5-arnd@arndb.de>
Signed-off-by: Corey Minyard <cminyard@mvista.com>