net: enetc: use EXPORT_SYMBOL_GPL for enetc_phc_index

enetc_phc_index is only used via symbol_get, which was only ever
intended for very internal symbols like this one.  Use EXPORT_SYMBOL_GPL
for it so that symbol_get can enforce only being used on
EXPORT_SYMBOL_GPL symbols.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>