nfp: use correct index to mask link state irq

We were using an incorrect define to get the irq vector number.
NFP_NET_CFG_LSC is a control BAR offset, LSC interrupt vector
index is called NFP_NET_IRQ_LSC_IDX.  For machines with less
than 30 CPUs this meant that we were disabling/enabling IRQ 0.
For bigger hosts we were just playing with the 31st RX/TX
interrupt.

Fixes: 0ba40af9 ("nfp: move link state interrupt request/free calls")
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>