staging: sep: Add fallthrough comment

This patch solves the warning "possible switch/case default not preceded by
break or fallthrough comment".

Signed-off-by: Andreea-Cristina Bernat <bernat.ada@gmail.com>
Acked-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: Peter P Waskiewicz Jr <peter.p.waskiewicz.jr@intel.com>