drivers: ethernet: tc6: Combine read chunks into continuous net buffer

Up till now the size of net buffer chunk was set to only 64B. This
approach was acceptable for IPv4 support as all headers would fit into
64B of allocated continuous memory.

With enabled support for IPv6 one would observe following errors when
Neighbor Discovery [ND] is performed:

net_pkt: Uncontiguous data cannot be linearized
net_ipv6_nd: DROP: NULL NA header
net_icmpv6: ICMPv6 handling failure (-5)

As some IPv6 headers span on multiple 64B net_pkt buffer instances.

To fix this error - the received chunks are stored to large enough single
net_pkt buffer fragment.

Signed-off-by: Stefan Bigler <linux@bigler.io>
Signed-off-by: Lukasz Majewski <lukma@denx.de>