[spinel] define spinel_prop_key_t and spinel_status_t as uint32_t (#4402)

This commit defines `spinel_prop_key_t` and `spinel_status_t` as
`uint32_t` instead of `enum` value. This allows these types to
take values not included in `enum` (e.g., vendor or experimental
values) and addresses "out-of-range enumeration value" compiler
warnings.

src/ncp/spinel.h

@@ -374,7 +374,7 @@
 extern "C" {
 #endif
 
-typedef enum
+enum
 {
     SPINEL_STATUS_OK                       = 0,  ///< Operation has completed successfully.
     SPINEL_STATUS_FAILURE                  = 1,  ///< Operation has failed for some undefined reason.
@@ -469,7 +469,9 @@ typedef enum
 
     SPINEL_STATUS_EXPERIMENTAL__BEGIN = 2000000,
     SPINEL_STATUS_EXPERIMENTAL__END   = 2097152,
-} spinel_status_t;
+};
+
+typedef uint32_t spinel_status_t;
 
 typedef enum
 {
@@ -1106,7 +1108,7 @@ enum
  *    Experimental |          2,000,000 - 2,097,151 | Experimental use only
  *
  */
-typedef enum
+enum
 {
     /// Last Operation Status
     /** Format: `i` - Read-only
@@ -3898,7 +3900,9 @@ typedef enum
 
     SPINEL_PROP_EXPERIMENTAL__BEGIN = 2000000,
     SPINEL_PROP_EXPERIMENTAL__END   = 2097152,
-} spinel_prop_key_t;
+};
+
+typedef uint32_t spinel_prop_key_t;
 
 // ----------------------------------------------------------------------------