dpaa2-switch: prevent ZERO_SIZE_PTR dereference when num_ifs is zero

In the Linux kernel, the following vulnerability has been resolved:

dpaa2-switch: prevent ZERO_SIZE_PTR dereference when num_ifs is zero

The driver allocates arrays for ports, FDBs, and filter blocks using kcalloc() with ethsw->sw_attr.num_ifs as the element count. When the device reports zero interfaces (either due to hardware configuration or firmware issues), kcalloc(0, ...) returns ZERO_SIZE_PTR (0x10) instead of NULL.

Later in dpaa2_switch_probe(), the NAPI initialization unconditionally accesses ethsw->ports[0]->netdev, which attempts to dereference ZERO_SIZE_PTR (address 0x10), resulting in a kernel panic.

Add a check to ensure num_ifs is greater than zero after retrieving device attributes. This prevents the zero-sized allocations and subsequent invalid pointer dereference.