Property bluetooth.protocol did make a distinction between A2DP sink and
source roles but on the contrary did not separate HFP roles (headset vs
gateway). For consistency, they should both behave similarly.
This automatically fixes another incosistency: the HFGW (or HSP) sink
was set to bluetooth.protocol="sco", while the source was set to "hsp".
There is no use for this distinction, since the protocol (including the
role) is the same.