Fix below build warning:

  CC      arch/arm/mach-omap2/hwspinlock.o
In file included from arch/arm/mach-omap2/hwspinlock.c:22:
include/linux/hwspinlock.h: In function '__hwspin_unlock':
include/linux/hwspinlock.h:121: warning: 'return' with a value, in function returning void
Signed-off-by: NAxel Lin <axel.lin@gmail.com>
Signed-off-by: NOhad Ben-Cohen <ohad@wizery.com>