topshim: hfp: Respond to AT+CNUM query

Send an OK response to HF to indicate that we have no subscriber info.
This is mandatory support for passing HFP/AG/NUM/BV-01-I.

Bug: 214149380
Tag: #floss
Test: HFP/AG/NUM/BV-01-I
Change-Id: If515d4385e1c7a2a4a6345f5bdc7199a42cf3eac