net: mvneta: fix handling of the Tx descriptor counter

net: mvneta: fix handling of the Tx descriptor counter

commit 0d63785c6b94b5d2f095f90755825f90eea791f5 upstream.

The mvneta controller provides a 8-bit register to update the pending
Tx descriptor counter. Then, a maximum of 255 Tx descriptors can be
added at once. In the current code the mvneta_txq_pend_desc_add function
assumes the caller takes care of this limit. But it is not the case. In
some situations (xmit_more flag), more than 255 descriptors are added.
When this happens, the Tx descriptor counter register is updated with a
wrong value, which breaks the whole Tx queue management.

This patch fixes the issue by allowing the mvneta_txq_pend_desc_add
function to process more than 255 Tx descriptors.

Fixes: 2a90f7e1d5d0 ("net: mvneta: add xmit_more support")
Signed-off-by: Simon Guinot <simon.guinot@sequanux.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>