drbd: cleanup: change "<= 0" to "== 0"

dt is unsigned so it's never less than zero.  We are calculating the
elapsed time, and that's never less than zero (unless there is a bug or
we invent time travel).  The comparison here is just to guard against
divide by zero bugs.

Signed-off-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: Philipp Reisner <philipp.reisner@linbit.com>