selftests/powerpc: Fix argument order to timer_sub()

Commit c814bf95 ("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: c814bf95 ("powerpc/selftests: Use timersub() for gettimeofday()")
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Madhavan Srinivasan <maddy@linux.ibm.com>
Link: https://patch.msgid.link/20241218114347.428108-1-mpe@ellerman.id.au