snd_uaudio(4): Fix string index computations for iFeature.

This allows the iFeature strings to be properly read by the snd_uaudio(4) driver,
when parsing the audio feature unit descriptors.

Submitted by: Zhichao1.Li@dell.com
Sponsored by: NVIDIA Networking

(cherry picked from commit 11f09b17fe2cf0b5489601aee548a06486f0b749)