pcm: Return unsigned int if error occurs by `read` or `write` system call

alsa-project/alsa-lib pull request #242 was opened from Korilakkuma:

`read` or `write` system call returns `-1` if error occurs.

Request URL : https://github.com/alsa-project/alsa-lib/pull/242 Patch URL : https://github.com/alsa-project/alsa-lib/pull/242.patch Repository URL: https://github.com/alsa-project/alsa-lib