staging: comedi: pcm_common: remove module

This module is built whenever CONFIG_COMEDI is enabled but it is
only used by the pcmmio and pcmuio drivers. The pcm_common module
consists of one exported function. Put a local copy of the function
in the pcmmio and pcmuio drivers.

This removes the need for the pcm_common module and the now unused
pcm_common.[ch] files can be deleted and removed from the Makefile.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Cc: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>