bcmgenet: fix Rx buffer corruption caused by lack of cache flush

bcmgenet: fix Rx buffer corruption caused by lack of cache flush

When bcmgenet complete to write Rx buffer with the DMA,
some U-Boot commands write data to the buffer directly.
Those write data will become dirty in CPU cache.
After this driver calls free_pkt to the buffer,
the buffer is assigned as the future Rx buffer.
At some point, if bcmgenet writes to a buffer with DMA
and CPU cache flushes dirty data to the buffer,
the buffer is corrupted.

This patch calls flush_dcache_range in free_pkt
to immediately flush the data written by U-Boot command
and prevent data corruption.

This issue can be reproduced using wget on Raspberry Pi4.
If wget receives data larger than
RX_BUF_LENGTH * RX_DESCS = 2048 * 256 bytes,
it will timeout due to data corruption.
In addition, if LOG_DEBUG is enabled in net/tcp.c,
the following error log is output.

TCP RX TCP xSum Error

Signed-off-by: Yasuharu Shibata <yasuharu.shibata@gmail.com>