drivers/misc/sgi-gru: add return on error

If the buffer is too small then return the error and in the process
remove the variables which became unused.

Signed-off-by: Sudip Mukherjee <sudip@vectorindia.org>
Acked-by: Dimitri Sivanich <sivanich@sgi.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>