net: mvpp2: fix hardcoded number of rx queues

There's a dedicated #define that indicates the number of rx queues per
port per cpu, this commit removes a harcoded use of that value

This doesn't fix any runtime bugs since the harcoded value matches the
expected value.

Signed-off-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Signed-off-by: David S. Miller <davem@davemloft.net>