lwm2m: Fix multi res inst, create flags and post write callback for SWMGMT

The multi resource and create flags were reversed, meaning that resources
were defined as multi resource but weren't being created by default. That
doesn't reflect the LWM2M Software Management, which specifies which
objects can be multi resource and which are mandatory.

The post write callbacks were assigned to the validate callbacks.

Signed-off-by: Michal Ciesielski <michal.m.ciesielski@voiapp.io>