net: dsa: sja1105: Make function sja1105_xfer_long_buf static

Fix sparse warnings:

drivers/net/dsa/sja1105/sja1105_spi.c:159:5: warning: symbol 'sja1105_xfer_long_buf' was not declared. Should it be static?
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: zhengbin <zhengbin13@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/dsa/sja1105/sja1105_spi.c

@@ -156,7 +156,7 @@ int sja1105_xfer_u32(const struct sja1105_private *priv,
  * must be sent/received. Splitting the buffer into chunks and assembling
  * those into SPI messages is done automatically by this function.
  */
-int sja1105_xfer_long_buf(const struct sja1105_private *priv,
+static int sja1105_xfer_long_buf(const struct sja1105_private *priv,
 				 sja1105_spi_rw_mode_t rw, u64 base_addr,
 				 void *packed_buf, u64 buf_len)
 {