ARM: vfp: rename last_VFP_context to vfp_current_hw_state

Rename the slightly confusing 'last_VFP_context' variable to be more
descriptive of what it actually is.  This variable stores a pointer
to the current owner's vfpstate structure for the context held in the
VFP hardware.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>