ice: remove unnecessary NULL checks before ptp_read_system_*

The ptp_read_system_prets and ptp_read_system_postts functions already
check for the NULL value of the ptp_system_timestamp structure pointer.
There is no need to check this manually in the ice driver code. Remove
the checks.

Reported-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>