wusbcore: add sysfs attribute for retry count

This patch adds a sysfs attribute for the wireless host controller
transaction retry count.  It also changes the default value from 15
retries to infinite retries because the driver currently does not handle
retry errors gracefully.

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