net/oic: make OC_MBUF_ENDPOINT use OS_MBUF_USRHDR
net/oic: make OC_MBUF_ENDPOINT use OS_MBUF_USRHDR

OC_MBUF_ENDPOINT() does the same thing as OS_MBUF_USRHDR().  Therefore,
it should just invoke the existing macro rather than duplicate it.

mlaz/mynewt-core,mlaz/mynewt-core,mlaz/mynewt-core,mlaz/mynewt-core,mlaz/mynewt-core