Staging: hv: use the correct #ifdef for x86-64

x86-64 needs a different config check.  Thanks to Hank for the debugging
to determine the fix for this.

Cc: Hank Janssen <hjanssen@microsoft.com>
Cc: Haiyang Zhang <haiyangz@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>