net: dsa: mv88e6xxx: shorten the locked section in mv88e6xxx_g1_atu_prob_irq_thread_fn()

As only the hardware access functions up til and including
mv88e6xxx_g1_atu_mac_read() called under the interrupt handler
need to take the chip lock, we release the chip lock after this call.
The follow up code that handles the violations can run without the
chip lock held.
In further patches, the violation handler function will even be
incompatible with having the chip lock held. This due to an AB/BA
ordering inversion with rtnl_lock().

Signed-off-by: Hans J. Schultz <netdev@kapio-technology.com>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>