bgmac: stop clearing DMA receive control register right after it is set

Current bgmac code initializes some DMA settings in the receive control
register for some hardware and then immediately clears those settings.
Not clearing those settings results in ~420Mbps *improvement* in
throughput; this system can now receive frames at line-rate on Broadcom
5871x hardware compared to ~520Mbps today.  I also tested a few other
values but found there to be no discernible difference in CPU
utilization even if burst size and prefetching values are different.

On the hardware tested there was no need to keep the code that cleared
all but bits 16-17, but since there is a wide variety of hardware that
used this driver (I did not look at all hardware docs for hardware using
this IP block), I find it wise to move this call up and clear bits just
after reading the default value from the hardware rather than completely
removing it.

This is a good candidate for -stable >=3.14 since that is when the code
that was supposed to improve performance (but did not) was introduced.

Signed-off-by: Andy Gospodarek <gospo@broadcom.com>
Fixes: 56ceecde1f29 ("bgmac: initialize the DMA controller of core...")
Cc: Hauke Mehrtens <hauke@hauke-m.de>
Acked-by: Hauke Mehrtens <hauke@hauke-m.de>
Signed-off-by: David S. Miller <davem@davemloft.net>