mesh: Take advantage of BT_MESH_NET_HDR_LEN define
mesh: Take advantage of BT_MESH_NET_HDR_LEN define

Avoid using hard-coded magic numbers.

X-Original-Commit: 6b68fdfc0fbd1808a12eef00a49b4952325a82e7

apache/mynewt-nimble,apache/mynewt-nimble,apache/mynewt-nimble,apache/mynewt-nimble,apache/mynewt-nimble