bluetooth: accept +VGM/+VGS unsolicited reply with '=' and ':' separator

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

Part-of: <pulseaudio/pulseaudio!521>
2 jobs for hsp-hfp-detect-gain-control in 2 minutes and 30 seconds (queued for 1 second)