This is likely the result of my cleanup of wr-servo, where I forgot
some pieces.

Greg collected some interesting logs in wrpc where the setpoint
was calculated wrongly. This fixes the thing and removes some redundancy.

I "git diff" order:

- avoid WR_UNINITIALIZED, really unused. Do what's needed at
  wr_servo_init() time

- adjust phase to 0 at init time, where s->cur_setpoint is set (we
  really should set and use in a single unified place)

- set delta_ms_prev before entering TRACK_PHASE, not earlier where it
  doesn't fit

- avoid extra checks of non-zero offset.seconds and
offset.nanoseconds, because they are checked at the beginning anyways.
Signed-off-by: Alessandro Rubini <rubini@gnudd.com>