net: lwm2m: return 4.04 when URI is not composed by digits

Modify zoap_options_to_path() to return error when URI contains
character other than digits and return 4.04 NOT FOUND to caller.

PATH such as "/1a/2/3" was treated as "/1/2/3" after parsring
which is incorrect.

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