drivers: hv: Fix wrong check for synic_event_page

The check for calling free_page() on hv_context.synic_event_page[cpu] is the
same for hv_context.synic_message_page[cpu], like a copy-paste error.

Signed-off-by: Felipe Pena <felipensp@gmail.com>
Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>