ASoC: mediatek: mtk-btcvsd: Fix an error handling path in 'mtk_btcvsd_snd_probe()'

If an error occurs after a successful 'of_iomap()' call, it must be undone
by a corresponding 'iounmap()' call, as already done in the remove
function.

While at it, remove the useless initialization of 'ret' at the beginning of
the function.

Fixes: 4bd8597d ("ASoC: mediatek: add btcvsd driver")
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Link: https://lore.kernel.org/r/0c2ba562c3364e61bfbd5b3013a99dfa0d9045d7.1622989685.git.christophe.jaillet@wanadoo.fr


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