gianfar: Fix Rx BSY error handling

The Rx BSY error interrupt indicates that a frame was
received and discarded due to lack of buffers, so it's
a rx ring overflow condition and has nothing to do with
with bad rx packets.  Use the right counter.

BSY conditions happen when the SoC is under performance
stress.  Doing *more* work in stress situations by trying
to schedule NAPI is not a good idea as the stressed system
becomes still more stressed.  The Rx interrupt is already
at work making sure the NAPI is scheduled.
So calling gfar_receive() here does not help.  This issue
was present since day 1.

Signed-off-by: Claudiu Manoil <claudiu.manoil@freescale.com>
Signed-off-by: David S. Miller <davem@davemloft.net>