Fix incorrect key polling in key calibration routine (misplaced semicolon)

[64kramsystem]

The key polling cycle in the key calibration routine has a misplaced semicolon, which prevented WaitVBL() to run on each poll (cycle).

I'm actually no expert 😬 - I've just compared the source project with the port, and had a look at the APIs involved. The calibration actually works fine either way.