Use port write_urbs_free mask rather than write_urb_busy field in struct
serial_port.

Compile-only tested.

Cc: Matthias Bruestle and Harald Welte <support@reiner-sct.com>
Signed-off-by: Johan Hovold <jhovold@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>