Revert "Bluetooth: Remove rfcomm_carrier_raised()"

Revert "Bluetooth: Remove rfcomm_carrier_raised()"

This reverts commit a0be612a9ff382579adb6bc73459c7e5c883fa34.

This is the first of a 3-patch revert, together with
Revert "Bluetooth: Always wait for a connection on RFCOMM open()" and
Revert "Bluetooth: Move rfcomm_get_device() before rfcomm_dev_activate()".

Commit 9b23da9a3c80115df5047cbd0850d70706d971ff,
"Bluetooth: Always wait for a connection on RFCOMM open()" open-codes
blocking on tty open(), rather than using the default behavior
implemented by the tty port.

The reasons for reverting that patch are detailed in that changelog;
this patch restores required functionality for that revert.

Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Tested-By: Alexander Holler <holler@ahsoftware.de>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>