net: thunderx: Program LMAC credits based on MTU

Programming LMAC credits taking 9K frame size by default is incorrect
as for an interface which is one of the many on the same BGX/QLM
no of credits available will be less as Tx FIFO will be divided
across all interfaces. So let's say a BGX with 40G interface and another
BGX with multiple 10G, bandwidth of 10G interfaces will be effected when
traffic is running on both 40G and 10G interfaces simultaneously.

This patch fixes this issue by programming credits based on netdev's MTU.
Also fixed configuring MTU to HW and added CQE counter for pkts which
exceed this value.

Signed-off-by: Sunil Goutham <sgoutham@cavium.com>
Signed-off-by: David S. Miller <davem@davemloft.net>