net: cavium: Fix build errors due to 'imply CAVIUM_PTP'

If CAVIUM_PTP is m and THUNDER_NIC_VF is y, build fails:

drivers/net/ethernet/cavium/thunder/nicvf_main.o: In function 'nicvf_remove':
nicvf_main.c:(.text+0x1f0): undefined reference to 'cavium_ptp_put'
drivers/net/ethernet/cavium/thunder/nicvf_main.o: In function `nicvf_probe':
nicvf_main.c:(.text+0x557c): undefined reference to 'cavium_ptp_get'

THUNDER_NIC_VF imply CAVIUM_PTP, which allow the config now,
Use IS_REACHABLE() to avoid the vmlinux link error for this case.

Reported-by: Hulk Robot <[e-mail]>
Fixes: def2fbffe62c ("kconfig: allow symbols implied by y to become m")
Signed-off-by: YueHaibing <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

drivers/net/ethernet/cavium/common/cavium_ptp.h

···
 	struct ptp_clock *ptp_clock;
 };
 
-#if IS_ENABLED(CONFIG_CAVIUM_PTP)
+#if IS_REACHABLE(CONFIG_CAVIUM_PTP)
 
 struct cavium_ptp *cavium_ptp_get(void);
 void cavium_ptp_put(struct cavium_ptp *ptp);