drivers/net/e1000e: fix sparse warning: Should it be static?

Impact: Make symbol static.

Fix this sparse warning:
  drivers/net/e1000e/82571.c:1229:5: warning: symbol 'e1000_check_for_serdes_link_82571' was not declared. Should it be static?

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