qlge: Fix build error without CONFIG_ETHERNET

Now if CONFIG_ETHERNET is not set, QLGE driver
building fails:

drivers/staging/qlge/qlge_main.o: In function `qlge_remove':
drivers/staging/qlge/qlge_main.c:4831: undefined reference to `unregister_netdev'

Reported-by: Hulk Robot <hulkci@huawei.com>
Fixes: 955315b0 ("qlge: Move drivers/net/ethernet/qlogic/qlge/ to drivers/staging/qlge/")
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>