USB: serial: keyspan_pda: fix write implementation

Fix stalled writes by checking the available buffer space after
requesting an unthrottle notification in case the device buffer is
already empty so that no notification is ever sent (e.g. when doing
single character writes).

This also means we can drop the room query from write() which was
conditioned on in_interrupt() and prevented writing using this driver
from atomic contexts (e.g. PPP).

Acked-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Johan Hovold <johan@kernel.org>