[spinel] make SendCommand public to allow calling without variable arguments (#10602)

Changed access of SendCommand to public to allow calling without
variable arguments. This is to address compilation error in
ot-br-posix repo NcpSpinel::ThreadErasePersistentInfo (while building
for OpenWrt v23.05.4 for a mips target)(error: 'args' may be used
uninitialized [-Werror=maybe-uninitialized])

src/lib/spinel/spinel_driver.hpp

@@ -180,6 +180,20 @@ public:
                         const char       *aFormat,
                         va_list           aArgs);
 
+    /*
+     * Sends a spinel command without arguments to the co-processor.
+     *
+     * @param[in] aCommand    The spinel command.
+     * @param[in] aKey        The spinel property key.
+     * @param[in] aTid        The spinel transaction id.
+     *
+     * @retval  OT_ERROR_NONE           Successfully sent the command through spinel interface.
+     * @retval  OT_ERROR_INVALID_STATE  The spinel interface is in an invalid state.
+     * @retval  OT_ERROR_NO_BUFS        The spinel interface doesn't have enough buffer.
+     *
+     */
+    otError SendCommand(uint32_t aCommand, spinel_prop_key_t aKey, spinel_tid_t aTid);
+
     /*
      * Sets the handler to process the received spinel frame.
      *
@@ -291,8 +305,6 @@ private:
                                    void          *aContext);
     void        HandleInitialFrame(const uint8_t *aFrame, uint16_t aLength, uint8_t aHeader, bool &aSave);
 
-    otError SendCommand(uint32_t aCommand, spinel_prop_key_t aKey, spinel_tid_t aTid);
-
     otError         CheckSpinelVersion(void);
     otError         GetCoprocessorVersion(void);
     otError         GetCoprocessorCaps(void);