[radio-spinel] always be ready to ready from socket (#3319)
[radio-spinel] always be ready to ready from socket (#3319)

This commit changes the logic in `RadioSpinel` such that the HDLC
socket is always added to read fd set in `UpdateFdSet()` replacing
the existing code where read was not allowed during an ongoing 15.4
frame transmission. Note that during a frame transmission, radio can
potentially receive a frame (e.g. during CSMA backoffs). Also other
spinel frames such as logs can be received during tx.
turon/openthread,erja-gp/openthread,lanyuwen/openthread,bukepo/openthread,chshu/openthread,srickardti/openthread,abtink/openthread,erja-gp/openthread,openthread/openthread,turon/openthread,librasungirl/openthread,chshu/openthread,lanyuwen/openthread,srickardti/openthread,librasungirl/openthread,lanyuwen/openthread,srickardti/openthread,erja-gp/openthread,turon/openthread,abtink/openthread,erja-gp/openthread,jwhui/openthread,lanyuwen/openthread,jwhui/openthread,srickardti/openthread,openthread/openthread,abtink/openthread,lanyuwen/openthread,jwhui/openthread,abtink/openthread,chshu/openthread,turon/openthread,chshu/openthread,librasungirl/openthread,chshu/openthread,jwhui/openthread,openthread/openthread,chshu/openthread,bukepo/openthread,lanyuwen/openthread,bukepo/openthread,bukepo/openthread,erja-gp/openthread,erja-gp/openthread,librasungirl/openthread,openthread/openthread