net; ethernet: ti: cpsw: move irq stuff under cpsw_common

The irq data are common for net devs in dual_emac mode. So no need to
hold these data in every priv struct, move them under cpsw_common.
Also delete irq_num var, as after optimization it's not needed.
Correct number of irqs to 2, as anyway, driver is using only 2,
at least for now.

Signed-off-by: Ivan Khoronzhuk <ivan.khoronzhuk@linaro.org>
Reviewed-by: Mugunthan V N <mugunthanvnm@ti.com>
Signed-off-by: David S. Miller <davem@davemloft.net>