i40e: fix i40e_ptp_adjtime when given a negative delta

Commit 0ac30ce4 ("i40e: fix up 32 bit timespec references",
2017-07-26) claims to be cleaning up references to 32-bit timespecs.

The actual contents of the commit make no sense, as it converts a call
to timespec64_add into timespec64_add_ns. This would seem ok, if (a) the
change was documented in the commit message, and (b) timespec64_add_ns
supported negative numbers.

timespec64_add_ns doesn't work with signed deltas, because the
implementation is based around iter_div_u64_rem. This change resulted in
a regression where i40e_ptp_adjtime would interpret small negative
adjustments as large positive additions, resulting in incorrect
behavior.

This commit doesn't appear to fix anything, is not well explained, and
introduces a bug, so lets just revert it.

Reverts: 0ac30ce4

 ("i40e: fix up 32 bit timespec references", 2017-07-26)
Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>