inky: Minor cleanups

[davidsansome]

• Use maxTxSize from conn.Limits
• Show an error instead of crashing on invalid pin names

[codecov-io]

Codecov Report

Merging #431 into master will decrease coverage by 0.04%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master     #431      +/-   ##
==========================================
- Coverage   63.55%   63.51%   -0.05%     
==========================================
  Files         117      117              
  Lines       12181    12187       +6     
==========================================
- Hits         7742     7740       -2     
- Misses       4241     4249       +8     
  Partials      198      198

Impacted Files	Coverage Δ
experimental/devices/inky/inky.go	0% <0%> (ø)	⬆️
devices/lepton/lepton.go	87.19% <0%> (-1.22%)	⬇️

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[maruel]

gohci

[maruel]

Thanks!