i40e: Fix off by one in i40e_dbg_command_write

We assume that the resulting buffer is zero terminated when we then
re-use it. The sscanf is limited to 512 bytes but needs to be 511
to allow for a terminator.

One of a set of problems noted by Jackie Chang

Signed-off-by: Alan Cox <alan@linux.intel.com>
Acked-by: Shannon Nelson <Shannon.nelson@intel.com>
Tested-by: Kavindya Deegala <kavindya.s.deegala@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>