net: stmmac: use readl_poll_timeout() function in init_systime()

The init_systime() function use an open coded of readl_poll_timeout().
Replace the open coded handling with the proper function.

Signed-off-by: Dejin Zheng <zhengdejin5@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>