ASoC: mmp-sspa: Remove the embedded struct ssp_device

The "serial port" it represents is actually a SPI controller -- it's not
clear why would the audio serial interface embed it. We're only using
the mmio_base and clk fields.

Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Link: https://lore.kernel.org/r/20200511210134.1224532-7-lkundrak@v3.sk


Signed-off-by: Mark Brown <broonie@kernel.org>