wusbcore: add sysfs attribute for DNTS count and interval

This patch adds a sysfs attribute for the wireless USB host controller
device notification transmit slot(DNTS) count and interval.  It also
changes the defaults from 16 slots in every MMC to a more reasonable 4
slots every 2ms.

Signed-off-by: Thomas Pugliese <thomas.pugliese@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>