net: airoha: Set REG_RX_CPU_IDX() once in airoha_qdma_fill_rx_queue()

net: airoha: Set REG_RX_CPU_IDX() once in airoha_qdma_fill_rx_queue()

It is not necessary to update REG_RX_CPU_IDX register for each iteration
of the descriptor loop in airoha_qdma_fill_rx_queue routine.
Move REG_RX_CPU_IDX configuration out of the descriptor loop and rely on
the last queue head value updated in the descriptor loop.

Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Link: https://patch.msgid.link/20260331-airoha-cpu-idx-out-off-loop-v1-1-75c66b428f50@kernel.org
Signed-off-by: Paolo Abeni <pabeni@redhat.com>