net: enetc: fix potential divide-by-zero when num_vsi is zero

In the Linux kernel, the following vulnerability has been resolved: net: enetc: fix potential divide-by-zero when num_vsi is zero For i.MX94 series, all the standalone ENETCs do not support SR-IOV, so pf->caps.num_vsi is zero. This leads to a divide-by-zero in enetc4_default_rings_allocation() when distributing rings among PF and VFs. Division by zero is undefined behavior in C. On ARM64, the UDIV/SDIV instructions silently return zero rather than raising an exception, so the issue does not cause a visible crash. However, relying on this behavior is incorrect and poses a cross-platform compatibility risk. Add an explicit check for num_vsi == 0 and return early after the PF's rings have been configured.