hfp: Handle extra CCWA event

hfp: Handle extra CCWA event

When a call is waitng, CCWA event is sent and call object
in state WAITING is created. on ReleaseAndAnswer it is
promoted to INCOMING and later to ACTIVE.
iPhones send an extra CCWA event when active call is ended.
This extra event is creating a second call object in state
WAITING. It is not possible to have two WAITING calls, but
previously waiting call was already promoted to INCOMING.
For a brief time we have two calls from the same number,
one INCOMING and one WAITING. Later WAITING one is removed.
As we cannot have a waiting and incoming call at the same
time, ignore CCWA when there is already an INCOMING call.

< \r\n+CIEV: 3,3\r\n
< \r\n+CIEV: 2,1\r\n
< \r\n+CIEV: 3,0\r\n
< \r\n+CCWA: "01234567890",129,1,"Me"\r\n
< \r\n+CIEV: 3,1\r\n
> AT+CLCC\r
< \r\n+CLCC: 1,0,0,0,0,"09876543210",129,"Me"\r\n
< \r\n+CLCC: 2,1,5,0,0,"01234567890",129,"Me"\r\n
< \r\nOK\r\n
< \r\n+CIEV: 2,0\r\n
< \r\n+CCWA: "01234567890",129,1,"Me"\r\n
< \r\n+CIEV: 2,1\r\n
< \r\n+CIEV: 3,0\r\n
> AT+CLCC\r
< \r\n+CLCC: 2,1,0,0,0,"01234567890",129,"Me"\r\n
< \r\nOK\r\n