net: ethernet: Add missing VENDOR to Cadence and Packet Engines symbols

The vendor guard Kconfig symbols for Cadence and Packet Engines use a
"NET_" prefix, while all other vendor guards use a "NET_VENDOR_"
prefix.  Hence make them consistent with the rest, and add the missing
trailing "S" for Packet Engines while at it.

As these options don't directly affect the kernel build, and default to
"y", this change has no impact on kernels built with existing
(def)configs.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: David S. Miller <davem@davemloft.net>