ASoC: rt5659: remove redundant assignment to variable idx

Variable idx is being assigned with a value that is never idx, it is
assigned a new value a couple of statements later. The assignment 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/20200208221529.37105-1-colin.king@canonical.com
Signed-off-by: Mark Brown <broonie@kernel.org>