net: mctp-i2c: invalidate flows immediately on TX errors

If we encounter an error on i2c packet transmit, we won't have a valid
flow anymore; since we didn't transmit a valid packet sequence, we'll
have to wait for the key to timeout instead of dropping it on the reply.

This causes the i2c lock to be held for longer than necessary.

Instead, invalidate the flow on TX error, and release the i2c lock
immediately.

Cc: Bonnie Lo <Bonnie_Lo@wiwynn.com>
Tested-by: Jerry C Chen <Jerry_C_Chen@wiwynn.com>
Signed-off-by: Jeremy Kerr <jk@codeconstruct.com.au>
Signed-off-by: David S. Miller <davem@davemloft.net>