[spinel] reset mState to kStateDisabled if recovering from kStateDisaled (#9450)

This commit changes openthread rcp recovery code to keep `mState` as
`kStateDisabled` if the state before recovery was
`kStateDisabled`. This is needed to avoid otbr-agent crash in the
scenario where rcp recovery is triggered before otbr initialization.

src/lib/spinel/radio_spinel_impl.hpp

@@ -2115,6 +2115,8 @@ template <typename InterfaceType> void RadioSpinel<InterfaceType>::RecoverFromRc
     switch (recoveringState)
     {
     case kStateDisabled:
+        mState = kStateDisabled;
+        break;
     case kStateSleep:
         break;
     case kStateReceive: