ili9341: st7789: fix SetScrollArea

[aykevl]

The existing code was broken in a few ways:

• It didn't use the correct operator precedence for the VSCRDEF VSA variable: it needed some extra parentheses to be correct.
• It used the configured height instead of the actual display height for calculating VSA, which is incorrect. TFA+VSA+BFA must always be exactly 320, even if a lower screen height is configured.
• If a lower than 320 pixel height is configured, the bottomFixedArea parameter applied to the whole 320 pixel screen height. Because this seems counter intuitive (and relies on properties of any given screen), I've changed it to work from the actual visible bottom of the screen (which may be smaller than 320 pixels). TODO: this doesn't take RowOffset into account, while it probably should.

I haven't fixed the st7735 implementation, because I didn't have example code on hand that would easily work on a st7735 screen. This is left as a TODO for the future.

Found while working on hardware scrolling support in https://github.com/aykevl/tinygl.

[aykevl]

I have a patch ready that will fix https://github.com/tinygo-org/drivers/issues/573 but it depends on this patch and a decision on #582.

[conejoninja]

It didn't break my code, but I don't have a screen of different size (320), I don't see any issue with the code itself, so ... merge?

[deadprogram]

Thanks for the corrections @aykevl and to @conejoninja for review/test. Now merging.