1.优化错误码，默认为负。

device/pwm.c

@@ -14,7 +14,7 @@
 
 static mr_err_t err_io_pwm_configure(mr_pwm_t pwm, mr_pwm_config_t config)
 {
-    return -MR_ERR_IO;
+    return MR_ERR_IO;
 }
 
 static void err_io_pwm_write(mr_pwm_t pwm, mr_off_t channel, mr_uint32_t duty)
@@ -61,7 +61,7 @@ static mr_err_t mr_pwm_ioctl(mr_device_t device, int cmd, void *args)
                 }
                 return ret;
             }
-            return -MR_ERR_INVALID;
+            return MR_ERR_INVALID;
         }
 
         case MR_DEVICE_CTRL_GET_CONFIG:
@@ -72,11 +72,11 @@ static mr_err_t mr_pwm_ioctl(mr_device_t device, int cmd, void *args)
                 *config = pwm->config;
                 return MR_ERR_OK;
             }
-            return -MR_ERR_INVALID;
+            return MR_ERR_INVALID;
         }
 
         default:
-            return -MR_ERR_UNSUPPORTED;
+            return MR_ERR_UNSUPPORTED;
     }
 }
 
@@ -88,7 +88,7 @@ static mr_err_t mr_pwm_read(mr_device_t device, mr_off_t pos, void *buffer, mr_s
 
     if (pos < 0)
     {
-        return -MR_ERR_INVALID;
+        return MR_ERR_INVALID;
     }
 
     while ((read_size += sizeof(*read_buffer)) <= size)
@@ -108,7 +108,7 @@ static mr_err_t mr_pwm_write(mr_device_t device, mr_off_t pos, const void *buffe
 
     if (pos < 0)
     {
-        return -MR_ERR_INVALID;
+        return MR_ERR_INVALID;
     }
 
     while ((write_size += sizeof(*write_buffer)) <= size)