USB: wusb: add wusb_phy_rate sysfs file to host controllers

Add the wusb_phy_rate sysfs file to Wireless USB host controllers.  This
sets the maximum PHY rate that will be used for all connected devices.

Signed-off-by: David Vrabel <david.vrabel@csr.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>