staging: Comedi: comedi_fops: Fixed the return error code

This patch fixes the checkpatch.pl warning:

WARNING: ENOSYS means 'invalid syscall nr' and nothing else

try_module_get fails when the reference count of the module is not
allowed to be incremented ,and hence -ENXIO is returned indicating
no device or address.

[IA - combined two of santhosh's changes to the error return value!]

Signed-off-by: santhosh pai <santhosh.pai88@yahoo.com>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>