net: stmmac: make some functions static

The functions dwmac4_dma_init_rx_chan, dwmac4_dma_init_tx_chan and
dwmac4_dma_init_channel do not need to be in global scope, so them
static.

Cleans up sparse warnings:
"symbol 'dwmac4_dma_init_rx_chan' was not declared. Should it be static?"
"symbol 'dwmac4_dma_init_tx_chan' was not declared. Should it be static?"
"symbol 'dwmac4_dma_init_channel' was not declared. Should it be static?"

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: David S. Miller <davem@davemloft.net>