add backport.h to follow upstream

An ALSA kernel API is changed and there were already some APIs changed
in upstream. According to this, this commit add backport.h to replace
the API with its alternatives for Linux 3.11-3.13.

See:
[alsa-devel] [RFC PATCH 00/24] Mandate to pass a device pointer at card creation
http://mailman.alsa-project.org/pipermail/alsa-devel/2014-February/072348.html