sparc64: rename virt_irq => irq - I

The generic irq support uses the term 'irq' for the
allocated irq number.
Fix it so sparc64 use the same term for an irq as the
generic irq support does.
For a naive reader this is less confusing.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: David S. Miller <davem@davemloft.net>