staging: wilc1000: del_key: use netdev private wilc instead of g_linux_wlan

Use netdev private data member wilc instead of global variable g_linux_wlan.

Signed-off-by: Glen Lee <glen.lee@atmel.com>
Signed-off-by: Tony Cho <tony.cho@atmel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>