uart.c: fix return value for invalid baudrates in uint2speed

This patch probably changes no functionality.

Before this patch the return value 0 was used for invalid baudrates in
uint2speed, but this is not a proper speed_t constant.
Instead the constant B0 is returned now, since on x86 and arm linux
architectures B0 is defined as 0.

Signed-off-by: Tapani Utriainen <tapani@technexion.com>
Signed-off-by: Brendan Le Foll <brendan.le.foll@intel.com>

src/uart/uart.c

@@ -110,7 +110,7 @@ uint2speed(unsigned int speed)
 #endif
         default:
             // if we are here, then an unsupported baudrate was selected.
-            return 0;
+            return B0;
     }
 }
 
@@ -364,7 +364,7 @@ mraa_uart_set_baudrate(mraa_uart_context dev, unsigned int baud)
 
     // set our baud rates
     speed_t speed = uint2speed(baud);
-    if (speed == 0)
+    if (speed == B0)
     {
         syslog(LOG_ERR, "uart%i: set_baudrate: invalid baudrate: %i", dev->index, baud);
         return MRAA_ERROR_INVALID_PARAMETER;