i40e: convert .adjfreq to .adjfine

The i40e driver currently implements the .adjfreq handler for frequency
adjustments. This takes the adjustment parameter in parts per billion. The
PTP core supports .adjfine which provides an adjustment in scaled parts per
million. This has a higher resolution and can result in more precise
adjustments for small corrections.

Convert the existing .adjfreq implementation to the newer .adjfine
implementation. This is trivial since it just requires changing the divisor
from 1000000000ULL to (1000000ULL << 16) in the mul_u64_u64_div_u64 call.

This improves the precision of the adjustments and gets us one driver
closer to removing the old .adjfreq support from the kernel.

Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Tested-by: Gurucharan <gurucharanx.g@intel.com> (A Contingent worker at Intel)
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>