rx: Make clock_Add correctly add to itself

rx: Make clock_Add correctly add to itself

With the existing clock_Add code, the following:
    struct clock a = {2, 800000};
    clock_Add(&a, &a);
gives a clock value of {6, 600000}, rather than the expected {5, 60000}.

This is because the ordering of instructions leads it to double count
the carry on the seconds field. Reorder the instructions so that the
carry is correctly applied.

Change-Id: Ia71b387ce521a11e4caf9ec200907efe1d2be8ff
Reviewed-on: http://gerrit.openafs.org/4864
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Reviewed-by: Jeffrey Altman <jaltman@openafs.org>
Tested-by: Jeffrey Altman <jaltman@openafs.org>