memory: gpmc: fix out of bounds read and dereference on gpmc_cs[]

Currently the array gpmc_cs is indexed by cs before it cs is range checked
and the pointer read from this out-of-index read is dereferenced. Fix this
by performing the range check on cs before the read and the following
pointer dereference.

Addresses-Coverity: ("Negative array index read")
Fixes: 9ed7a776 ("ARM: OMAP2+: Fix support for multiple devices on a GPMC chip select")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Reviewed-by: Tony Lindgren <tony@atomide.com>
Link: https://lore.kernel.org/r/20210223193821.17232-1-colin.king@canonical.comSigned-off-by: Krzysztof Kozlowski <krzk@kernel.org>

drivers/memory/omap-gpmc.c

@@ -1009,8 +1009,8 @@ EXPORT_SYMBOL(gpmc_cs_request);
 
 void gpmc_cs_free(int cs)
 {
-	struct gpmc_cs_data *gpmc = &gpmc_cs[cs];
-	struct resource *res = &gpmc->mem;
+	struct gpmc_cs_data *gpmc;
+	struct resource *res;
 
 	spin_lock(&gpmc_mem_lock);
 	if (cs >= gpmc_cs_num || cs < 0 || !gpmc_cs_reserved(cs)) {
@@ -1018,6 +1018,9 @@ void gpmc_cs_free(int cs)
 		spin_unlock(&gpmc_mem_lock);
 		return;
 	}
+	gpmc = &gpmc_cs[cs];
+	res = &gpmc->mem;
+
 	gpmc_cs_disable_mem(cs);
 	if (res->flags)
 		release_resource(res);