ep93xx: clock: Don't use plain integer as NULL pointer

Fix sparse warning:
arch/arm/mach-ep93xx/clock.c:210:35: sparse: sparse: Using plain integer as NULL pointer

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Alexander Sverdlin <alexander.sverdlin@gmail.com>
Link: https://lists.01.org/hyperkitty/list/kbuild-all@lists.01.org/thread/TLFJ6D7WGMDJSQ6XK7UZE4XR2PLRZJSV/
Signed-off-by: Arnd Bergmann <arnd@arndb.de>

arch/arm/mach-ep93xx/clock.c

@@ -209,7 +209,7 @@ static int ep93xx_mux_determine_rate(struct clk_hw *hw,
 				struct clk_rate_request *req)
 {
 	unsigned long rate = req->rate;
-	struct clk *best_parent = 0;
+	struct clk *best_parent = NULL;
 	unsigned long __parent_rate;
 	unsigned long best_rate = 0, actual_rate, mclk_rate;
 	unsigned long best_parent_rate;