USB: serial: keyspan_pda: clean up comments and whitespace

Clean up comment style, remove some stale or redundant comments and drop
superfluous white space.

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