Set ENET_BD_BDU in I.MX FEC controller

This commit properly sets the ENET_BD_BDU flag once the emulated FEC controller
has finished processing the last descriptor. This is done for both transmit
and receive descriptors.

This allows the QNX 7.0.0 BSP for the Sabrelite board (which can be
found at http://blackberry.qnx.com/en/developers/bsp) to properly
control the FEC. Without this patch, the BSP ethernet driver will never
re-use FEC descriptors, as the unset ENET_BD_BDU flag will cause
it to believe that the descriptors are still in use by the NIC.

Note that Linux does not appear to use this field at all, and is
unaffected by this patch.

Without this patch, QNX will think that the NIC is still processing its
transaction descriptors, and won't send any more data over the network.

For reference:

On page 1192 of the I.MX 6DQ reference manual revision (Rev. 5, 06/2018),
which can be found at https://www.nxp.com/products/processors-and-microcontrollers/arm-based-processors-and-mcus/i.mx-applications-processors/i.mx-6-processors/i.mx-6quad-processors-high-performance-3d-graphics-hd-video-arm-cortex-a9-core:i.MX6Q?&tab=Documentation_Tab&linkline=Application-Note

the 'BDU' field is described as follows for the 'Enhanced transmit
buffer descriptor':

'Last buffer descriptor update done. Indicates that the last BD data has been updated by
uDMA. This field is written by the user (=0) and uDMA (=1).'

The same description is used for the receive buffer descriptor.

Signed-off-by: Aaron Hill <aa1ronham@gmail.com>
Message-id: 20190805142417.10433-1-aaron.hill@alertinnovation.com
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

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