net/macb: clear tx/rx completion flags in ISR

At least in the cadence IP core on the Xilinx Zynq SoC the TCOMP/RCOMP flags
are not auto-cleaned. As these flags are evaluated, they need to be cleaned.

Signed-off-by: Steffen Trumtrar <s.trumtrar@pengutronix.de>
Cc: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>