Xen / ACPI / processor: Remove unneeded NULL check

Before xen_acpi_processor_enable() is called, struct acpi_processor *pr is
allocated in xen_acpi_processor_add() and checked if it's NULL, so no need
to check again when passed to xen_acpi_processor_enable(), just remove it.

Signed-off-by: Hanjun Guo <hanjun.guo@linaro.org>
CC: Boris Ostrovsky <boris.ostrovsky@oracle.com>
CC: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>