net: mvpp2: add FCA RXQ non occupied descriptor threshold

The firmware needs to monitor the RX Non-occupied descriptor
bits for flow control to move to XOFF mode.
These bits need to be unmasked to be functional, but they will
not raise interrupts as we leave the RX exception summary
bit in MVPP2_ISR_RX_TX_MASK_REG clear.

Signed-off-by: Stefan Chulski <stefanc@marvell.com>
Acked-by: Marcin Wojtas <mw@semihalf.com>
Signed-off-by: David S. Miller <davem@davemloft.net>