tunnel: Add to nw_tos bits instead of replacing them in tnl_port_send().

We normally only add 1-bits to wc->masks for datapath flow matching
purposes, never removing them.  In this case, the bits that get set to
zero will be set back to 1 later on in the function, so this does not fix
any actual bug, but the principle of only setting to 1, not to 0, seems
sound to me.

Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Justin Pettit <jpettit@nicira.com>