Prevent premature device free in interface_claim_device

Prevent premature device free in interface_claim_device

interface_set_device_config can trigger a device free (for example
if the device is here only present in a bridge), which renders dev
invalid and leads to segfault. Add a lock to prevent this and
clean-up the code for readability.

Signed-off-by: Gino Peeters <[email protected]>
Signed-off-by: Hans Dedecker <[email protected]>
Signed-off-by: Felix Fietkau <[email protected]>