net: mvpp2: rework RXQ interrupt group initialization for PPv2.2

This commit adjusts how the MVPP2_ISR_RXQ_GROUP_REG register is
configured, since it changed between PPv2.1 and PPv2.2.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: David S. Miller <davem@davemloft.net>