carl9170: remove set but not used variable 'udev'

Fixes gcc '-Wunused-but-set-variable' warning:

drivers/net/wireless/ath/carl9170/usb.c: In function carl9170_usb_disconnect:
drivers/net/wireless/ath/carl9170/usb.c:1110:21:
 warning: variable udev set but not used [-Wunused-but-set-variable]

It is not use since commit feb09b29 ("carl9170:
fix misuse of device driver API")

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Acked-by: Christian Lamparter <chunkeey@gmail.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>