staging: vt6656: Use proper target pointer in memcpy()
The coverity scanner marked these two memcpy()'s as causing a buffer
overflow in CIDs 142743 and 142744. This is due the h_dest member of
struct ethhdr being used as a target (size ETH_ALEN) in memcpy, but the
copy is of size ETH_HLEN. However, the intention here seems to be to
copy the entire ethernet header. Make that clear by specifying the
proper destination buffer.
Also remove the unnecessary casts of the source argument.