net: gianfar: Implement rx_missed_errors counter

Devices with RMON support has a 16-bit RDRP counter.  It provides: "Receive
dropped packets counter. Increments for frames received which are streamed
to system but are later dropped due to lack of system resources."

To handle more than 2^16 dropped packets, a carry bit in CAR1 register is
set on overflow, so we enable irq when this is set, extending the counter
to 2^64 for handling situations where lots of packets are missed (e.g.
during heavy network storms).

Signed-off-by: Esben Haabendal <esben@geanix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>