net: thunderx: Fix issue with IRQ namimg

This patch fixes a regression caused by previous commit
when irq name exceeds 20 byte array if interface's name
size is large.

Fixes: e4126213 ("net: thunderx: Use netdev's name for naming VF's interrupts")
Signed-off-by: Sunil Goutham <sgoutham@cavium.com>
Signed-off-by: David S. Miller <davem@davemloft.net>