net: mvneta: replace magic numbers by existing macros

Some literal values are actually already defined by macros, so let's use
them.

[gregory.clement@free-electrons.com: split intial commit in two
individual changes]
Signed-off-by: Dmitri Epshtein <dima@marvell.com>
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Signed-off-by: David S. Miller <davem@davemloft.net>