I2S: Use 8 MHz oscillator source if 48MHz divider does not fit in 8 bits

[sandeepmistry]

Fixes sketch from @andrewjfox in #310 (comment)

The divider can have a max resolution of 8-bits since we are using generator 3:

These changes use the 8 MHz oscillator as the clock source instead of the 48 MHz DFLL if the divider can't fit in 8-bits.

[ArduinoBot]

✅ Build completed.

⬇️ Build URL: http://downloads.arduino.cc/PR/samd/package_samd-b190_index.json

ℹ️ To test this build:

1. Open the Preferences of the Arduino IDE.
2. Add the Build URL above in the Additional Boards Manager URLs field, and click OK.
3. Open the Boards Manager (menu Tools->Board->Board Manager...)
4. Install Arduino SAMD core - Pull Request I2S: Use 8 MHz oscillator source if 48MHz divider does not fit in 8 bits #317
5. Select one of the boards under SAMD Pull Request I2S: Use 8 MHz oscillator source if 48MHz divider does not fit in 8 bits #317 in Tools->Board menu
6. Compile/Upload as usual

[andrewjfox]

Just tested and getting correct clock freq for 8 bits @ 8kHz sampling

[sandeepmistry]

@andrewjfox great, thanks for trying and your feedback!