selftests/powerpc: Fix argument order to timer_sub()

Commit c814bf958926 ("powerpc/selftests: Use timersub() for
gettimeofday()"), got the order of arguments to timersub() wrong,
leading to a negative time delta being reported, eg:

test: gettimeofday
tags: git_version:v6.12-rc5-409-gdddf291c3030
time = -3.297781
success: gettimeofday

The correct order is minuend, subtrahend, which in this case is end,
start. Which gives:

test: gettimeofday
tags: git_version:v6.12-rc5-409-gdddf291c3030-dirty
time = 3.300650
success: gettimeofday

Fixes: c814bf958926 ("powerpc/selftests: Use timersub() for gettimeofday()")
Signed-off-by: Michael Ellerman <[e-mail]>
Signed-off-by: Madhavan Srinivasan <[e-mail]>
Link: https://patch.msgid.link/[e-mail]

tools/testing/selftests/powerpc/benchmarks/gettimeofday.c

···
 		gettimeofday(&tv_end, NULL);
 	}
 
-	timersub(&tv_start, &tv_end, &tv_diff);
+	timersub(&tv_end, &tv_start, &tv_diff);
 
 	printf("time = %.6f\n", tv_diff.tv_sec + (tv_diff.tv_usec) * 1e-6);