ptp: fix struct member comment for do_aux_work

The do_aux_work callback had documentation in the structure comment
which referred to it as "do_work".

Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Cc: Richard Cochran <richardcochran@gmail.com>
Acked-by: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>