intel_scu_ipc: Protect dev member assignment on ->remove()

Protect the dev member assignment in ->remove() since user may potentially call
unbind from a sysfs even if the driver is built-in. The latter might be racy
with ongoing SCU communication.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Darren Hart <dvhart@linux.intel.com>