staging: unisys: unify channel attributes into visorbus_main.c

The code in channel_attr.[ch] only creates sysfs files and is called
only in visorbus_main.c.  This code should be unified into
visorbus_main.c.   There are some functions that have been made static.

Signed-off-by: Prarit Bhargava <prarit@redhat.com>
Signed-off-by: Benjamin Romer <benjamin.romer@unisys.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>