ofctl_v1_0: match_to_str() does not convert nw_tos correctly

The match_to_str() function in ryu/lib/ofctl_v1_0.py does not convert
nw_tos correctly.  As a result, we cannot obtain nw_tos value of any
flow entry by invoking get_flow_stats()

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>