gianfar: fix phc index build failure

This patch fixes a build failure introduced in commit 66636287


("gianfar: Support the get_ts_info ethtool method."). Not only was a
global variable inconsistently named, but also it was not exported as
it should have been.

This fix is also needed in stable version 3.5.

Signed-off-by: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>