USB: serial: usb_wwan: fix TIOCSSERIAL jiffies conversions

The port close_delay and closing_wait parameters set by TIOCSSERIAL are
specified in jiffies and not milliseconds.

Add the missing conversions so that the TIOCSSERIAL works as expected
also when HZ is not 1000.

Fixes: 02303f73 ("usb-wwan: implement TIOCGSERIAL and TIOCSSERIAL to avoid blocking close(2)")
Cc: stable@vger.kernel.org      # 2.6.38
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Johan Hovold <johan@kernel.org>