hfpmodem: Fix release-and-swap without +CIEV

Some phones do not send the corresponding call state update (+CIEV)
after a successful release-and-swap operation (AT+CHLD=1).

This has been observed with a Nokia 500, while testing ReleaseAndSwap()
while an active and a held call exist:

ofonod[20414]: > AT+CLCC\r
ofonod[20414]: < \r\n+CLCC: 1,0,1,0,0,"<number1>",145\r\n
ofonod[20414]: < \r\n+CLCC: 2,0,0,0,0,"<number2>",145\r\n
ofonod[20414]: < \r\nOK\r\n
ofonod[20414]: > AT+CHLD=1\r
ofonod[20414]: < \r\nOK\r\n

After this, no +CIEV is received, but the call has been hung up.

The proposed approach to solve this consists of using AT+CLCC, unless
a call release has been received within a specific time period.

The result fixes the problem as can be seen below:

ofonod[20847]: < \r\n+CLCC: 1,0,1,0,0,"<number1>",145\r\n
ofonod[20847]: < \r\n+CLCC: 2,0,0,0,0,"<number2>",145\r\n
ofonod[20847]: < \r\nOK\r\n
ofonod[20847]: > AT+CHLD=1\r
ofonod[20847]: < \r\nOK\r\n
ofonod[20847]: > AT+CLCC\r
ofonod[20847]: < \r\n+CLCC: 1,0,0,0,0,"<number1>",145\r\n
ofonod[20847]: < \r\nOK\r\n
ofonod[20847]: < \r\n+CIEV: 5,2\r\n
ofonod[20847]: < \r\n+CIEV: 5,0\r\n

0 files changed, 0 insertions, 0 deletions