Don't do soft-disconnect if it's previously done. Likewise, don't do
soft-connect if the device is currently connected and running. It would
break normal operation.
Currently the caller of pullup() (udc's sysfs soft_connect) only checks
if it had initiated disconnect to prevent repeating soft-disconnect. It
doesn't check for soft-connect. To be safe, let's keep the check here
regardless whether the udc core is fixed.