ptp: uapi: change _IOW to IOWR in PTP_SYS_OFFSET_EXTENDED definition

The ioctl command is read/write (or just read, if the fact that user space
writes n_samples field is ignored).

Signed-off-by: Eugene Syromiatnikov <esyr@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>