eth: asix88179: Print packet length properly

Debug printf used '%u' to print size_t variable.
This caused warnings on 64-bit machines.

Signed-off-by: Mateusz Kulikowski <mateusz.kulikowski@gmail.com>
Acked-by: Marek Vasut <marex@denx.de>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>