dr_range bitfield should be unsigned integer

If the value is an integer, the 4th bit is used for sign, so you can store
values upto 7 only whereas the datarate values could go upto 15. That's why
we need to turn this to an unsigned integer so that the last bit can also be used.

Author: Hasnain Virk

features/lorawan/lorawan_types.h

@@ -275,7 +275,7 @@ typedef union {
     /**
      * Byte-access to the bits.
      */
-    int8_t value;
+    uint8_t value;
     /**
      * The structure to store the minimum and the maximum datarate.
      */
@@ -288,7 +288,7 @@ typedef union {
          * The allowed ranges are region-specific.
          * Please refer to \ref DR_0 to \ref DR_15 for details.
          */
-        int8_t min : 4;
+        uint8_t min : 4;
         /**
          * The maximum data rate.
          *
@@ -297,7 +297,7 @@ typedef union {
          * The allowed ranges are region-specific.
          * Please refer to \ref DR_0 to \ref DR_15 for details.
          */
-        int8_t max : 4;
+        uint8_t max : 4;
     } fields;
 } dr_range_t;