net: lwm2m: don't load credentials on plaintext context

Since lwm2m_load_tls_credentials(ctx) will assume that the ctx has a
valid security object assigned to it, it should not be called at all
when ctx.use_dtls == false.

This solves a major bug where LwM2M comms are DTLS encrypted but FOTA is
allowed to be plain-text.

Signed-off-by: Benjamin Lindqvist <benjamin@eub.se>