[radio-spinel] verify host is connected to an RCP (#5822)
[radio-spinel] verify host is connected to an RCP (#5822)

This commit addresses an issue in `RadioSpinel::Init()` to correctly
verify that device is connected to an RCP (i.e., capability list
includes `SPINEL_CAP_CONFIG_RADIO`) and if not then exit (`DieNow()`)
with `OT_EXIT_RADIO_SPINEL_INCOMPATIBLE` code.

It also simplifies the checking of capabilities for log stream support
(`SPINEL_CAP_OPENTHREAD_LOG_METADATA`) by changing the code to get and
process the `SPINEL_PROP_RADIO_CAPS` list only once from `IsRcp()`.
abtink/openthread,chshu/openthread,abtink/openthread,chshu/openthread,srickardti/openthread,openthread/openthread,bukepo/openthread,jwhui/openthread,jwhui/openthread,bukepo/openthread,abtink/openthread,chshu/openthread,abtink/openthread,srickardti/openthread,jwhui/openthread,srickardti/openthread,bukepo/openthread,srickardti/openthread,openthread/openthread,chshu/openthread,openthread/openthread,chshu/openthread,chshu/openthread,openthread/openthread,jwhui/openthread,bukepo/openthread