i40e: Fix ethtool coalesce settings

This patch fixes the i40e_set_coalesce function to allow 0 as a disable
value.  Also, added message to user about invalid value and provides valid
range.

Change-ID: I6c9ff11a9861f2045bd543745a3d132999ffbbd8
Signed-off-by: Carolyn Wyborny <carolyn.wyborny@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>