powerpc/selftests: Use timersub() for gettimeofday()

Use timersub() function to simplify the code.

Reported-by: Zeal Robot <zealci@zte.com.cn>
Signed-off-by: ye xingchen <ye.xingchen@zte.com.cn>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220816105106.82666-1-ye.xingchen@zte.com.cn