queue/wbt_lat_usec controls both the stored WBT latency target and the
effective WBT enable state.

The old no-op check skipped updates whenever the converted latency
matched the stored min_lat_nsec. That check ignored whether the current
WBT state already matched the state requested by the write. For a queue
disabled by default, attempting to enable WBT by writing the default
value through sysfs could return success while the enable state was left
unchanged.

Treat a write as a no-op only when both the stored latency and the
effective WBT enabled state already match the converted value.

Signed-off-by: Guzebing <guzebing1612@gmail.com>