Since commit cf32deec ("stmmac: add tx_skbuff_dma to save descriptors used by PTP"),
the struct dma_desc *p in stmmac_tx_clean was not used at all.

This patch remove this dead code.
Signed-off-by: Corentin Labbe <clabbe.montjoie@gmail.com>
Acked-by: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Signed-off-by: David S. Miller <davem@davemloft.net>