ehea: only register irq after setting up ports

If we receive an interrupt too early before we set up ports in the probe
function, there won't be any port ready to handle it.

Only registering the irq after the ports are setup fixes the problem,
and works fine without losing any interrupts.

This causes crashes in some situations:

[c000000f7ff7fd60] d000000008e223f0 .ehea_neq_tasklet+0x78/0x148 [ehea]
[c000000f7ff7fe00] c0000000000b6cac .tasklet_hi_action+0xdc/0x210
[c000000f7ff7fea0] c0000000000b7cc8 .__do_softirq+0x178/0x300
[c000000f7ff7ff90] c000000000022694 .call_do_softirq+0x14/0x24
[c000000f68ee7900] c000000000010e04 .do_softirq+0xec/0x110
[c000000f68ee79a0] c0000000000b789c .irq_exit+0xac/0xe0
[c000000f68ee7a20] c0000000000110bc .do_IRQ+0x114/0x2a8
[c000000f68ee7ae0] c00000000000553c hardware_interrupt_entry+0x18/0x1c

Signed-off-by: David S. Miller <davem@davemloft.net>