net: mvpp2: adapt mvpp2_defaults_set() to PPv2.2

This commit modifies the mvpp2_defaults_set() function to not do the
loopback and FIFO threshold initialization, which are not needed for
PPv2.2.

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