ARM: vfp: rename check_exception to vfp_hw_state_valid

Rename this branch to more accurately reflect why its taken, rather
than what the following code does.  It is the only caller of this code.
This helps to clarify following changes, yet this change results in no
actual code change.

Document the VFP hardware state at the target of this branch.

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