ice: Use FIELD_PREP for timestamp values

Instead of using shifts and casts, use FIELD_PREP after reading 40b
timestamp values.

Rename a couple defines for better clarity and consistency.

Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: Karol Kolacinski <karol.kolacinski@intel.com>
Tested-by: Pucha Himasekhar Reddy <himasekharx.reddy.pucha@intel.com> (A Contingent worker at Intel)
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>