net: mvpp2: add comments about smp_processor_id() usage

A previous commit modified a number of smp_processor_id() used in
migration-enabled contexts into get_cpu/put_cpu sections. However, a few
smp_processor_id() calls remain in the driver, and this commit adds
comments explaining why they can be kept.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: David S. Miller <davem@davemloft.net>