fix(module: stepper): fixed stepper disabled bug

[sWhite01111]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/NG-ZORRO/ng-zorro-antd-mobile/blob/master/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[x] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Documentation content changes
[ ] Application (the showcase website) / infrastructure changes
[ ] Other... Please describe:

What is the current behavior?

Issue Number: N/A

What is the new behavior?

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

[Anberm]

I am not in favor of such a modification. In disable true to false, input still cannot be input. Similarly, disable value does not need to be determined in ngOnChanges, because this time it does not affect _upDisabled & _downDIsabled

[codecov]

Codecov Report

Merging #405 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #405      +/-   ##
==========================================
+ Coverage   89.14%   89.15%   +<.01%     
==========================================
  Files         321      321              
  Lines        7087     7089       +2     
  Branches      582      584       +2     
==========================================
+ Hits         6318     6320       +2     
  Misses        590      590              
  Partials      179      179

Impacted Files	Coverage Δ
components/stepper/stepper.component.ts	97.47% <100%> (+0.04%)	⬆️

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