net: mediatek: Drop unneeded dependency on NET_VENDOR_MEDIATEK

The whole block is protected by "if NET_VENDOR_MEDIATEK", so there is
no need for individual driver config symbols to duplicate this
dependency.

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