fib_trie: Fix the description of pos and bits

1) Fix one typo: s/tn/tp/
2) Fix the description about the "u" bits.

Signed-off-by: Xunlei Pang <xlpang@redhat.com>
Acked-by: Alexander Duyck <alexander.h.duyck@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>