net: lwm2m: update internal API select_writer() and select_reader()

As per review of PR #5893, this is a follow up patch to update
select_writer() API to match the behavior of select_reader() API.

Quote from OMA-TS-LightweightM2M-V1_0_1-20170704-A. 8.2.5
"An Object Instance or Resource is Read by sending a CoAP GET to the
 corresponding path. The response includes the value in the
 corresponding Plain Text, Opaque, TLV or JSON format according to
 the specified Content-Format (see section 6.4).The request MAY
 specify an Accept option containing the preferred Content-Format
 to receive. When the specified Content-Format is not supported by
 the LwM2M Client, the request MUST be rejected."

Therefore, we do not attempt to assign a content-format when the
requested one is not supported. Instead, we return an error code.

0 is returned when reader or writer has been successfully selected by
select_reader() or select_writer()

Signed-off-by: Robert Chou <robert.ch.chou@acer.com>