net: lwm2m: Prevent infinite loop in do_write_op_tlv function

In case unsopported TLV type or malformed packet is received, the
`do_write_op_tlv` function will end up in an infinite loop. Prevent that
by returning an error code in case it does not recognize TLV type.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>