Staging: comedi: comedi_fops.c: Fixed coding style issue

Fixed following checkpatch.pl warnings
WARNING: Prefer WRITE_ONCE(<FOO>, <BAR>) over ACCESS_ONCE(<FOO>) = <BAR>

WARNING: Prefer READ_ONCE(<FOO>) over ACCESS_ONCE(<FOO>)

Signed-off-by: Shyam Saini <mayhs11saini@gmail.com>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>