[radio-spinel] fix `GetNextTid()` (#7516)
[radio-spinel] fix `GetNextTid()` (#7516)

This commit updates `GetNextTid()` in `RadioSpinel` so that it
searches for the next unused TID. The current implementation only
checked the `mCmdNextTid` and assumed that if it was in-use, then all
other TIDs were also in-use. This may not be valid since a TID may be
in-use for a pending radio tx in `mTxRadioTid` while TIDs after that
are still available.

This should help address the situation where while waiting for a
pending radio tx (holding on to `mTxRadioTid`) 15 other radio API
calls happen (each consuming a TID) causing the `mCmdNextTid` to wrap
around back to `mTxRadioTid`.
jwhui/openthread,abtink/openthread,openthread/openthread,srickardti/openthread,openthread/openthread,srickardti/openthread,srickardti/openthread,jwhui/openthread,openthread/openthread,abtink/openthread,abtink/openthread,srickardti/openthread,jwhui/openthread,abtink/openthread,jwhui/openthread,openthread/openthread