Two parameters are added to allow using only i2s-in/out-data pin.
This is necessary when i2so-bck and i2so-ws are repurposed
especially because they overlap with SPI GPIO.
Fixes#6066
Preserve any existing sample rate for the I2S unit when performing an
`i2s_begin`. If nothing has ever been set, default to 44.1KHz as
before.