net: netcp: store network statistics in 64 bits

Previously the network statistics were stored in 32 bit variable
which can cause some stats to roll over after several minutes of
high traffic. This implements 64 bit storage so larger numbers
can be stored.

Signed-off-by: Michael Scherban <m-scherban@ti.com>
Signed-off-by: Murali Karicheri <m-karicheri2@ti.com>
Signed-off-by: Sekhar Nori <nsekhar@ti.com>
Signed-off-by: David S. Miller <davem@davemloft.net>