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 <peeters.gino@gmail.com>
Signed-off-by: Hans Dedecker <dedeckeh@gmail.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>