netfilter: nft_compat: don't truncate ethernet protocol type to u8

Use u16 for protocol and then cast it to __be16

>> net/netfilter/nft_compat.c:140:37: sparse: incorrect type in assignment (different base types)
   net/netfilter/nft_compat.c:140:37:    expected restricted __be16 [usertype] ethproto
   net/netfilter/nft_compat.c:140:37:    got unsigned char [unsigned] [usertype] proto
>> net/netfilter/nft_compat.c:351:37: sparse: incorrect type in assignment (different base types)
   net/netfilter/nft_compat.c:351:37:    expected restricted __be16 [usertype] ethproto
   net/netfilter/nft_compat.c:351:37:    got unsigned char [unsigned] [usertype] proto

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Arturo Borrero Gonzalez <arturo.borrero.glez@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>