ASoC: rt5665: remove redundant assignment to variable idx

The variable idx is being initialized with a value that is never
read and it is being updated later with a new value. The
initialization is redundant and can be removed.

Addresses-Coverity: ("Unused value")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Link: https://lore.kernel.org/r/20190705075303.14692-1-colin.king@canonical.com
Signed-off-by: Mark Brown <broonie@kernel.org>