ptp: gianfar: Use high resolution frequency method.

This patch depends on commit d8d26354 ("ptp: Introduce a high
resolution frequency adjustment method.")

The gianfar devices offer a frequency resolution of about 0.46 ppb
(depends on actual value of tmr_add, for the calculation assumed
0x80000000). This patch lets users of the device benefit from the increased
frequency resolution when tuning the clock. Thanks to the rounding the
maximum error between the requested frequency and the applied frequency
will then be about 0.23 ppb.

Tested on a v3.3.8 kernel on a real gianfar device. Verified compilation
on net-next (currently at v4.9-rc5).

Signed-off-by: Ulrik De Bie <ulrik.debie-os@e2big.org>
Signed-off-by: David S. Miller <davem@davemloft.net>