Implement workaround for BCM5719/BCM5720 TX hang.
The read DMA request logic operation is based on having sufficient
available space in the transmit data buffer (TXMBUF) before a read
DMA can be requested.  There are four read DMA channels that use
the TXMBUF, and the logic checks if the available free space in the
TXMBUF is large enough for all the data in the four Send Buffers
for which buffer descriptors have been fetched.  The Enable_Request
signal is asserted only if the free TXMBUF space is larger than the
sum of the four DMA length registers.  The power-up default value
of BGE_RDMA_LSO_CRPTEN_CTRL register bit 25 (bit 21 on BCM5720) is
zero, which selects the DMA length registers to connect to the
input of the adder block.  The DMA length registers are
asynchronously reset following BCM5719/BCM5720 power-up, and due to
the lack of synchronous deassertion of the length registers reset
signal these resisters may contain uninitialized values following
the reset deassertion.
In the case of the failure the uninitialized DMA length register
values added up to more than the TXMBUF size, which prevented the
assertion of the Enable_Request signal and any subsequent read DMA
to start.  This lockup condition is the root cause of failing to
generate any transmit traffic.

To workaround the issue, select alternate output of multiplexers
and transmit the first four Ethernet frames. This overwrites the
DMA length registers with valid values.

Reported by:	Geans Pin <geanspin@broadcom.com>
Reviewed by:	Geans Pin <geanspin@broadcom.com>