bluetooth: accept +VGM/+VGS unsolicited reply with '+' and '=' separator
requested to merge igor.v.kovalenko/pulseaudio:pr/handle-both-separators-in-gain-unsolicited-replies into hsp-hfp-detect-gain-control
HFP specs states both '=' and ':' should be accepted as a valid separator for +VGM and +VGS unsolicited result codes.
This change is cherry-picked from Rodrigo Araujo's work here https://lists.freedesktop.org/archives/pulseaudio-discuss/2017-September/028820.html