net: lib: lwm2m: use correct format specifier for LOG_ERR

Use correct format specifier for LOG_ERR in lwm2m_obj_device.c. The
previously used format specifier of %u was correct for 32 bit systems
but would produce a build warning for 64 bit systems.

Fixes #66441

Signed-off-by: Daniel DeGrasse <daniel.degrasse@nxp.com>