misc: xilinx_sdfec: Prevent integer overflow in xsdfec_table_write()

The checking here needs to handle integer overflows because "offset" and
"len" come from the user.

Fixes: 20ec628e ("misc: xilinx_sdfec: Add ability to configure LDPC")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Michal Simek <michal.simek@xilinx.com>
Reviewed-by: Dragan Cvetic <dragan.cvetic@xilinx.com>
Link: https://lore.kernel.org/r/20190821071122.GD26957@mwanda


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>