Make clock setup explicit instead of using the convoluted asf4 macro setup.
enable_clock_generator():
- Add GCLK_GENCTRL_OE to stick with the current setup.
- Handle divisor larger than 31 for generator 2
- Change the source argument so it can take the GCLK_GENCTRL_SRC_XXXX_Val
macros without casting to uint8_t.
This patch should not introduce any functional changes except keeping
GCLK_GENCTRL_OE enabled when the I2S clock is enabled.
