i40e: Fix NULL pointer dereference in i40e_dbg_dump_desc

When trying to dump VFs VSI RX/TX descriptors
using debugfs there was a crash
due to NULL pointer dereference in i40e_dbg_dump_desc.
Added a check to i40e_dbg_dump_desc that checks if
VSI type is correct for dumping RX/TX descriptors.

Fixes: 02e9c290 ("i40e: debugfs interface")
Signed-off-by: Sylwester Dziedziuch <sylwesterx.dziedziuch@intel.com>
Signed-off-by: Norbert Zulinski <norbertx.zulinski@intel.com>
Signed-off-by: Mateusz Palczewski <mateusz.palczewski@intel.com>
Tested-by: Gurucharan G <gurucharanx.g@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>