[ALSA] snd_pcm_format_name() is no longer exported

Modules: PCM Midlevel,USB generic driver

Because snd_pcm_format_name() function is used only for informational
purposes, it is no longer exported from the PCM midlevel to reduce
space and dependency. usbaudio module shows only numeric value for format.

Signed-off-by: Jaroslav Kysela <perex@suse.cz>