netdev-tc-offloads: Use customary types for buffer.

netdev-tc-offloads: Use customary types for buffer.

This function uses local array set_buff[] to store Netlink attributes.
It declares set_buff as an array of character pointers, which is a strange
type for a buffer of non-character-pointer objects.  In OVS it is
customary to use an ofpbuf with a stub of uint64_t objecs (to ensure
proper alignment, otherwise uint8_t would be more usual).  This commit
changes to that more usual form.

Signed-off-by: Ben Pfaff <blp@ovn.org>