gianfar: Only process completed frames

If the LAST bit is not set in the RxBD, it's possible we're processing
an incomplete frame, which is bad.  While we're at it, add a constant
for the error bitmask, so the whole if-clause fits on one line,
and is more legible.

Signed-off-by: Dai Haruki <dai.haruki@freescale.com>
Signed-off-by: Andy Fleming <afleming@freescale.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>