ip/tunnel: Simplify and unify tos printing

ip/tunnel: Simplify and unify tos printing

For ip tunnels tos can be 0 when not configured, 1 when
inherited from encapsulated packet and rest specifying
diffserv (rfc2474) or tos (rfc1349) bits. It is stored
in packet tos/diffserv field and returned in tos
netlink attribute to userspace.

Simplify and unify tos printing by using print_0xhex()
and print_string() instead of fprintf() to output values.

Signed-off-by: Serhey Popovych <serhe.popovych@gmail.com>
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>