staging: comedi: serial2002: use proper errno

checkpatch.pl reports:
WARNING: ENOSYS means 'invalid syscall nr' and nothing else

Change the error code to -ENOTTY.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>