iio: Remove a cast in iio-test-format which is no longer required

KUnit's EXPECT macros no longer typecheck as stringently, so casting the
result of strcmp() is now unnecessary.

Signed-off-by: David Gow <davidgow@google.com>
Reviewed-by: Brendan Higgins <brendanhiggins@google.com>
Acked-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>