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."

Change-Id: I2d759bd3d65f2adb9a364d40ffe23d6197c6375b