x86/vRTC: minor adjustment to reads from index port

x86/vRTC: minor adjustment to reads from index port

Whether to handle this shouldn't depend on the present value of the
index register. Since the handling is done outside of the lock anyway,
pull it out into the sole caller and drop the no longer needed function
parameter.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Roger Pau Monné <roger.pau@citrix.com>