Fix smw driver

[simonzihlmann]

I found this driver not working by default. Here are the changes proposed to make it working.

@FarBo or @jenshnielsen could you please have a look at it? And maybe also @lankes-fzj. Thanks.

[codecov-commenter]

Codecov Report

Merging #111 (a4cf8c7) into master (8cc993a) will decrease coverage by 0.07%.
The diff coverage is 0.00%.

@@            Coverage Diff            @@
##           master    #111      +/-   ##
=========================================
- Coverage    6.99%   6.92%   -0.08%     
=========================================
  Files          97      97              
  Lines       12261   12261              
=========================================
- Hits          858     849       -9     
- Misses      11403   11412       +9     

Impacted Files	Coverage Δ
...es_contrib_drivers/drivers/RohdeSchwarz/SMW200A.py	0.00% <0.00%> (ø)
qcodes_contrib_drivers/_version.py	43.36% <0.00%> (-3.23%)	⬇️

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