Adjust the configured homing speed so that it always results in a
speed that is an even number of mcu ticks per step. This ensures that
the code can always get good step compression during homing, which is
important as the entire homing operation must be able to fit within
the mcu's move queue. This fixes some "move queue empty" mcu shutdown
errors that could occur when the Z step distance was an unusual size.
Signed-off-by: Kevin O'Connor <kevin@koconnor.net>