Fixes reception of multicast custom Ethernet frames

[evpopov]

Description

When ipconfigETHERNET_DRIVER_FILTERS_FRAME_TYPES is disabled, the stack relies on eConsiderFrameForProcessing() to filter out frames.
If ipconfigPROCESS_CUSTOM_ETHERNET_FRAMES is enabled, the stack should receive unsupported Ethernet frames.
The current code does function properly for unicast and broadcast frames, however it filters out custom multicast frames. One example of such a frame is a PTP frame with a destination of 01:1B:19:00:00:00
This PR fixes the issue by explicitly allowing all unsupported frames when ipconfigPROCESS_CUSTOM_ETHERNET_FRAMES is enabled.

@HTRamsey, If I understand your intent correctly, you meant to to have the second filter stage of eConsiderFrameForProcessing() only drop packets. My PR changes eReturn to eProcessBuffer before breaking out of the filter which may be going against your original design.
The reason I took that approach is so to have a very simply path for these custom frames and avoid having to carve out a multicast exception further down in the function.

Test Steps

#define ipconfigETHERNET_DRIVER_FILTERS_FRAME_TYPES    ipconfigDISABLE
#define ipconfigPROCESS_CUSTOM_ETHERNET_FRAMES        ipconfigENABLE

With those settings, multicast frames with an EtherTYPE other than ARP, IPv4, pr IPv6 get dropped instead of being passed to eApplicationProcessCustomFrameHook()

Related Issue

May have been caused by #1100 but I'm not sure.

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