Add explicit type casting to fix build error

[DeflateAwning]

Fixes Issue #116.

Not tested on non-ESP32 devices yet. You may need to use some sort of macro if other MCUs' APIs aren't compatible with this change.

[DeflateAwning]

Please merge this fix.

For anyone stumbling upon this issue, you can add this to your platformio.ini for now (until they merge my fix):

lib_deps =
    https://github.com/DeflateAwning/Adafruit_BusIO.git#7fd30d0bf87e75ac9117d290a8d9ee77787c7335

[ladyada]

thanks!