drivers: net: Force LAN865x to start received frames at first chunk word

Zephyr's network stack has issues with network IP header split across
fragments.
To alleviate this problem, the frame would be now aligned to first byte
of the chunk. This would ensure that the header is stored at one network
buffer fragment (as we explicitly set its size to 64B).

Signed-off-by: Lukasz Majewski <lukma@denx.de>