net: vmxnet3: Fix incorrect rss_key_len in RSS mode

[WK-Tseng]

net: vmxnet3: Fix incorrect rss_key_len in RSS mode

The issue is located in the init_port_start function within ff_dpdk_if.c.
When setting the RSS mode for the net_vmxnet3 device,
the code was explicitly passing a fixed key length of rss_key_len = 40.

This incorrect value triggers the following DPDK error message:

"Ethdev port_id=0 invalid RSS key len: 40, valid value: 0"

Since the net_vmxnet3 does not need to set a custom RSS key during this initialization phase,
the correct value for the key length in this context is 0.