serial-parsers: don't echo-remove strange call start/end URCs

Some Sierra devices omit the leading <CR> from call start/end URCs
like NO CARRIER and CONNECT, which caused the echo-removal code to
remove from the response buffer because the leading <CR><LF> did
not exist. That can break call control and hangup handling on
those devices.

Signed-off-by: Dan Williams <dan@ioncontrol.co>

src/tests/test-at-serial-port.c

@@ -46,6 +46,8 @@ static const EchoRemovalTest echo_removal_tests[] = {
     { "echo echo\r\nthis is valid\r\nand so is this", "\r\nthis is valid\r\nand so is this" },
     { "\r\nthis is valid\r\nand so is this", "\r\nthis is valid\r\nand so is this" },
     { "\r\nthis is valid\r\nand so is this\r\n", "\r\nthis is valid\r\nand so is this\r\n" },
+    { "\nNO CARRIER\r\n", "\nNO CARRIER\r\n" },
+    { "\nCONNECT\r\n", "\nCONNECT\r\n" },
 };
 
 static const ParseResponseTest parse_ok_tests[] = {