Softuart module setup function parameters bug fix

[vsky279]

Fixes no reported issue.

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The following code is failing thought it was inline with documentation:

s = softuart.setup(115200, 3, nil)
s = softuart.setup(115200, 3)

The first call was failing as the third argument had to be a number or none. nil was not acccepted (but should be).
The second call failed as softuart did not accept to be setup rx only.

[nwf]

LGTM

[galjonsfigur]

Also looks good to me - when I was making #3104 I somehow missed this. Thanks for fixing this! Those kind of bugs and regressions could easily be found by simple tests - I wish I have time to write them especially in the lights of PR like #3324