ASoC: nau8540: remove redundant variable osrate

Variable osrate is being assigned but is never used hence it is
redundant and can be removed.

Cleans up clang warning:
warning: variable 'osrate' set but not used [-Wunused-but-set-variable]

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Mark Brown <broonie@kernel.org>