ARM: socfpga: PM: Drop useless check for PM_SUSPEND_STANDBY

As SoCFPGA uses the standard suspend_valid_only_mem() for its
platform_suspend_ops.valid() callback, its platform_suspend_ops.enter()
callback will never be called with state equal to PM_SUSPEND_STANDBY.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Tony Lindgren <tony@atomide.com>
Acked-by: Dinh Nguyen <dinguyen@kernel.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>