Fix I2S kernel clock source for STM32F4

[elagil]

The I2S driver for STM32F4 (and other series?) currently assumes the SPI peripheral clock to be the I2S kernel clock. This is not the case, because in I2S mode, the SPI peripheral is supplied by the I2S PLL.

RM0368, p. 585

Current implementation:

embassy/embassy-stm32/src/i2s.rs

Line 496 in b127ca9

//let pclk = unsafe { get_freqs() }.plli2s1_q.unwrap();

This leads to incorrect calculation of clock scaling factors in the I2S driver and thus incorrect I2S clocks.

[elagil]

The same is true for STM32F7 series devices, and probably others with combined SPI/I2S.

[elagil]

See #3716 for a fix for F4 series devices only.