staging: usbip: fix sparse warnings regarding endianness

The wHubCharacteristics field in usb_hub_descriptor structure is __le16
so there is no need for cast. The cpu_to_le16 returns a __le16 type for
a u16 type. Used cpu_to_le16 to silence last sparse error.

drivers/staging/usbip/vhci_hcd.c:223:35: warning: incorrect type in assignment (different base types)
drivers/staging/usbip/vhci_hcd.c:223:35:    expected restricted __le16 [usertype] wHubCharacteristics
drivers/staging/usbip/vhci_hcd.c:223:35:    got unsigned short [unsigned] [usertype] <noident>
drivers/staging/usbip/vhci_hcd.c:351:34: warning: incorrect type in assignment (different base types)
drivers/staging/usbip/vhci_hcd.c:351:34:    expected unsigned short [unsigned] [short] [usertype] <noident>
drivers/staging/usbip/vhci_hcd.c:351:34:    got restricted __le16 [usertype] <noident>
drivers/staging/usbip/vhci_hcd.c:352:34: warning: incorrect type in assignment (different base types)
drivers/staging/usbip/vhci_hcd.c:352:34:    expected unsigned short [unsigned] [short] [usertype] <noident>
drivers/staging/usbip/vhci_hcd.c:352:34:    got restricted __le16 [usertype] <noident>
drivers/staging/usbip/vhci_hcd.c:540:36: warning: restricted __le16 degrades to integer

Signed-off-by: Teodora Baluta <teobaluta@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>