Staging: comedi: proc: Warn if unable to create proc entry

The proc entry is not essential for the comedi system as
evident by the support for !CONFIG_PROC_FS. So for failure
to create, just warn and continue loading.

Signed-off-by: Cheah Kok Cheong <thrust73@gmail.com>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>