net: ethernet: ti: cpts: fix overflow check period

The CPTS drivers uses 8sec period for overflow checking with
assumption that CPTS retclk will not exceed 500MHz. But that's not
true on some TI platforms (Kesytone 2). As result, it is possible that
CPTS counter will overflow more than once between two readings.

Hence, fix it by selecting overflow check period dynamically as
max_sec_before_overflow/2, where
 max_sec_before_overflow = max_counter_val / rftclk_freq.

Cc: John Stultz <john.stultz@linaro.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Grygorii Strashko <grygorii.strashko@ti.com>
Acked-by: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>