net: mscc: ocelot: stop returning IRQ_NONE in ocelot_xtr_irq_handler

Since the xtr (extraction) IRQ of the ocelot switch is not shared, then
if it fired, it means that some data must be present in the queues of
the CPU port module. So simplify the code.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
(cherry picked from commit f833ca29

)
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
[Xulin: Original patch taken from
https://github.com/vladimiroltean/linux/commits/felix-dsa-tag-vlan-stable]
Signed-off-by: Xulin Sun <xulin.sun@windriver.com>