net: lwm2m: Rename timer object instance create function

Rename timer object instance create funciton `timer_create` to fix a
name collision regression with a POSIX function in `timer.h`. The issue
was introduced with commit 73a637ed when
first including`timer.h` into `lwm2ms.h`.

Signed-off-by: Marc Lasch <mlasch@mailbox.org>