usbnet: ipheth: break up NCM header size computation

Originally, the total NCM header size was computed as the sum of two
vaguely labelled constants. While accurate, it wasn't particularly clear
where they were coming from.

Use sizes of existing NCM structs where available. Define the total
NDP16 size based on the maximum amount of DPEs that can fit into the
iOS-specific fixed-size header.

This change does not fix any particular issue. Rather, it introduces
intermediate constants that will simplify subsequent commits.
It should also make it clearer for the reader where the constant values
come from.

Cc: stable@vger.kernel.org # 6.5.x
Signed-off-by: Foster Snowhill <forst@pen.gy>
Reviewed-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>