update BABYSTEP_MULTIPLICATOR_Z assert

[ellensp]

Description

BABYSTEP_MULTIPLICATOR_Z has two modes
If BABYSTEP_MILLIMETER_UNITS is defined the value must be less or equal to 0.1
else (ie step mode) the value needs to be a non zero integer,

BABYSTEP_MILLIMETER_UNITS has a static assert to check if the value is valid.
step mode did not.

This lead to #27438
Where the user set

//#define BABYSTEP_MILLIMETER_UNITS       // Specify BABYSTEP_MULTIPLICATOR_(XY|Z) in mm instead of micro-steps
  #define BABYSTEP_MULTIPLICATOR_Z  0.01       // (steps or mm) Steps or millimeter distance for each Z babystep

Causing baby stepping to not work.

I updated the description of the original assert to make it clear that BABYSTEP_MILLIMETER_UNITS was set.
I added a second assert to test for integers values for step mode.

Requirements

#define BABYSTEPPING
#define BABYSTEP_MILLIMETER_UNITS or //#define BABYSTEP_MILLIMETER_UNITS
#define BABYSTEP_MULTIPLICATOR_Z {assorted values}

Benefits

Less confused users

Related Issues

[BUG] Babystepping not working on BTT SKR Mini E3 V3.0 #27438