sim: Change TLV tag from u8 to u16

Change the type of the TLV tag from an 8-bit unsigned value (followed by
8 bits of padding) to a 16-bit value.  Because we are storing the tag as
little endian, the encoding will end up the same, but this will later
allow us to add more tags.

Signed-off-by: David Brown <david.brown@linaro.org>