wiseconnect: Avoid symbols conflicts in PSRAM driver

Symbols "control" and "config" are very generic and may conflict with
the user application.

In addition, they are not used outside of sl_si91x_psram.c. So they can
be declared with static qualifier.

Signed-off-by: Jérôme Pouiller <jerome.pouiller@silabs.com>

Author: jerome-pouiller

wiseconnect/components/device/silabs/si91x/mcu/drivers/unified_api/src/sl_si91x_psram.c

@@ -138,8 +138,8 @@ extern RSI_UDMA_HANDLE_T udmaHandle0;
 extern uint32_t dma_rom_buff0[30];
 
 static volatile struct xferContextType ctx;
-RSI_UDMA_CHA_CONFIG_DATA_T control;
-RSI_UDMA_CHA_CFG_T config;
+static RSI_UDMA_CHA_CONFIG_DATA_T control;
+static RSI_UDMA_CHA_CFG_T config;
 
 /*******************************************************************************
  *********************   LOCAL FUNCTION PROTOTYPES   ***************************