Staging: iio/resolver: Fix prefer pr_warn over pr_warning in ad2s1210.c

This patch fixes following checkpatch.pl warning
WARNING: Prefer pr_warn(... to pr_warning(...

Signed-off-by: Monam Agarwal <monamagarwal123@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>