HFP: Fix missing OK for AT+CLCC in virtual call

When AT+CLCC is received from HF while we're in virtual call mode there
is only non-zero index call sent without final zero index call. This
makes AT command response incomplete since no OK is sent.

From HFP specification, section 4.31.1:
"The AG shall always send OK response to HF."

https://android-review.googlesource.com/#/c/88384/