mgmt: mcumgr: smp: Fix custom payload size

Fixes an issue when using custom payloads where the size was still
using the zcbor buffer instead of the size of the network buffer

Signed-off-by: Jamie McCrae <jamie.mccrae@nordicsemi.no>
(cherry picked from commit 13ac8fe1f51faf050e0527d2c6e24583cb833a08)
1 file changed