usb: core: Remove unnecessary memset()

register_root_hub() calls memset() setting usb_dev->bus->devmap.
devicemap to 0 during hcd probe function (usb_hcd_pci_probe). But
in previous function which is also the procedure of usb_hcd_pci_probe(),
usb_bus_init() already initialized bus->devmap calling memset().
Furthermore, register_root_hub() is called only once in kernel.
So, calling memset() which resets usb_bus->devmap.devicemap in
register_root_hub() is redundant.

Signed-off-by: Suwan Kim <suwan.kim027@gmail.com>
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>