staging: wilc1000: remove varialbe sdio_set_default_speed

This patch removes sdio_set_default_speed of wilc_wlan_io_func_t which is
not used anymore and also remove struct sdio since it is empty.

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