drivers/net/wan/z85230.c: fix sparse warnings: un-EXPORT symbols

The symbols are only references within the translation unit they are
defined in, so un-EXPORT them und make them 'static'.

Fix this sparse warnings:

  drivers/net/wan/z85230.c:604:25: warning: symbol 'z8530_dma_sync' was not declared. Should it be static?
  drivers/net/wan/z85230.c:613:25: warning: symbol 'z8530_txdma_sync' was not declared. Should it be static?

Signed-off-by: Hannes Eder <hannes@hanneseder.net>
Signed-off-by: David S. Miller <davem@davemloft.net>