staging: comedi: make class and file operations static

A couple of global variables in "comedi_fops.c" are only referenced from
that .c file and can be declared 'static'.  Also remove them from
"comedi_fops.h" where they are declared 'extern'.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>