net: mvpp2: fix MVPP21_ISR_RXQ_GROUP_REG definition

The MVPP21_ISR_RXQ_GROUP_REG register is not indexed by rxq, but by
port, so we fix the parameter name accordingly. There are no
functional changes.

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