ixgbe: fix use of uninitialized padding

This patch is resolving Coverity hits where padding in a structure could
be used uninitialized.

- Initialize fwd_cmd.pad/2 before ixgbe_calculate_checksum()

- Initialize buffer.pad2/3 before ixgbe_hic_unlocked()

Signed-off-by: Emil Tantilov <emil.s.tantilov@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>