nrf52840: ieee802154: better error check on busy

Co-authored-by: Tyler Potyondy <77175673+tyler-potyondy@users.noreply.github.com>

chips/nrf52840/src/ieee802154_radio.rs

@@ -1351,9 +1351,7 @@ impl<'a> kernel::hil::radio::RadioData<'a> for Radio<'a> {
     ) -> Result<(), (ErrorCode, &'static mut [u8])> {
         if self.state.get() == RadioState::OFF {
             return Err((ErrorCode::OFF, buf));
-        } else if self.tx_buf.is_some() {
-            // tx_buf is only occupied when a transmission is underway. This
-            // check insures we do not interrupt an ongoing transmission.
+        } else if self.busy() {
             return Err((ErrorCode::BUSY, buf));
         } else if buf.len() < radio::PSDU_OFFSET + frame_len + radio::MFR_SIZE {
             // Not enough room for CRC