uart.c: make use of no_bus_mux flag

This patch makes use of no_bus_mux flag in mraa_board_t to skip
unnessary mux checks.

Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Signed-off-by: Brendan Le Foll <brendan.le.foll@intel.com>

src/uart/uart.c

@@ -152,6 +152,7 @@ mraa_uart_init(int index)
         return NULL;
     }
 
+    if (!plat->no_bus_mux) {
         int pos = plat->uart_dev[index].rx;
         if (pos >= 0) {
             if (plat->pins[pos].uart.mux_total > 0) {
@@ -171,6 +172,7 @@ mraa_uart_init(int index)
                 }
             }
         }
+    }
 
     mraa_uart_context dev = mraa_uart_init_raw((char*)plat->uart_dev[index].device_path);
     if (dev == NULL) {