drivers/char: fix handling cable re-plug in XHCI console driver

drivers/char: fix handling cable re-plug in XHCI console driver

When cable is unplugged, dbc_ensure_running() correctly detects this
situation (DBC_CTRL_DCR flag is clear), and prevent sending data
immediately to the device. It gets only queued in work ring buffers.
When cable is plugged in again, subsequent dbc_flush() will send the
buffered data.
But there is a corner case, where no subsequent data was buffered in the
work buffer, but a TRB was still pending. Ring the doorbell to let the
controller re-send them. For console output it is rare corner case (TRB
is pending for a very short time), but for console input it is very
normal case (there is always one pending TRB for input).

Extract doorbell ringing into separate function to avoid duplication.

Signed-off-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>
Acked-by: Jan Beulich <jbeulich@suse.com>