arptables-nft: fix decoding of hlen on bigendian platforms

arptables-nft: fix decoding of hlen on bigendian platforms

The existing test fail with:
extensions/libarpt_standard.t: ERROR: line 2 (cannot find: arptables -I INPUT -s 192.168.0.1)

... because hlen is 0 instead of expected "6".
The rule is correct, i.e. this is a decode/display bug: arp_hlen is
specified as 'unsigned short' instead of uint8_t.

On LSB systems, this doesn't matter but on MSB the value then is '0x600'
instead of '0x006' which becomes 0 when assignment to the u8 header field.

Signed-off-by: Florian Westphal <fw@strlen.de>
Acked-by: Phil Sutter <phil@nwl.cc>