iio:dummy: Drop set but unused variable len.

Not sure what the thinking was here, as lost to history, but the
variable is clearly not used so get rid of it.

Warning seen with clang W=1 tests (may be present with other compilers
and build options).

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Link: https://lore.kernel.org/r/20211128172445.2616166-11-jic23@kernel.org