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

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.

Fixes: 2d673b0e2f8d ("net: enetc: add standalone ENETC support for i.MX94")
Signed-off-by: Wei Fang <wei.fang@nxp.com>
Reviewed-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Link: https://patch.msgid.link/20260624072726.1238903-1-wei.fang@oss.nxp.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>