ptp: fix format string mismatch in ptp_sysfs.c

Fix format string mismatch in ptp_sysfs.c. Use %u for unsigned int.

Fixes: 73f37068

 ("ptp: support ptp physical/virtual clocks conversion")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Yangbo Lu <yangbo.lu@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>