rndis_wlan: cleanup: rename and remove local pointers

Mixed use of 'dev' and 'usbdev' for usbnet pointer can be confusing. So changing all 'usbnet *dev' to 'usbnet *usbdev'.

Also remove 'net_device *net' pointer from 'rndis_wext_bind' as 'usbdev->net' were already used where needed.

Signed-off-by: Jussi Kivilinna <jussi.kivilinna@mbnet.fi>
Signed-off-by: John W. Linville <linville@tuxdriver.com>