ARM: 7873/1: vfp: clear vfp_current_hw_state for dying cpu

The CPU_DYING notifier is called by cpu stopper task which
does not own the context held in the VFP hardware. Calling
vfp_force_reload() has no effect.
Replace it with clearing vfp_current_hw_state.

Change-Id: Ib93963fffdea784d2903c04d6d617a5cd884a8ce
Signed-off-by: Yuanyuan Zhong <zyy@motorola.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>