net: mvneta: Optimize rx path for small frame

For small frame reuse the phys_addr variable instead of accessing the
uncacheable value in the rx descriptor.

Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Tested-by: Marcin Wojtas <mw@semihalf.com>
Signed-off-by: David S. Miller <davem@davemloft.net>