ASoC: rsnd: adg: Fix BRG typos

"BRG" stands for "Baud Rate Generator", but is frequently misspelled as
"RBG".

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Link: https://lore.kernel.org/r/ac6365c17861d71fbc89d823089db4aafdb763ed.1676470202.git.geert+renesas@glider.be


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