net: ethernet: ti: cpts: fix ethtool output when no ptp_clock registered

The CPTS driver registers PTP PHC clock when first netif is going up and
unregister it when all netif are down. Now ethtool will show:
 - PTP PHC clock index 0 after boot until first netif is up;
 - the last assigned PTP PHC clock index even if PTP PHC clock is not
registered any more after all netifs are down.

This patch ensures that -1 is returned by ethtool when PTP PHC clock is not
registered any more.

Fixes: 8a2c9a5 ("net: ethernet: ti: cpts: rework initialization/deinitialization")
Signed-off-by: Grygorii Strashko <grygorii.strashko@ti.com>
Acked-by: Richard Cochran <richardcochran@gmail.com>
Link: https://lore.kernel.org/r/20201224162405.28032-1-grygorii.strashko@ti.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/ethernet/ti/cpts.c

@@ -599,6 +599,7 @@ void cpts_unregister(struct cpts *cpts)
 
 	ptp_clock_unregister(cpts->clock);
 	cpts->clock = NULL;
+	cpts->phc_index = -1;
 
 	cpts_write32(cpts, 0, int_enable);
 	cpts_write32(cpts, 0, control);
@@ -784,6 +785,7 @@ struct cpts *cpts_create(struct device *dev, void __iomem *regs,
 	cpts->cc.read = cpts_systim_read;
 	cpts->cc.mask = CLOCKSOURCE_MASK(32);
 	cpts->info = cpts_info;
+	cpts->phc_index = -1;
 
 	if (n_ext_ts)
 		cpts->info.n_ext_ts = n_ext_ts;