staging: wilc1000: remove function pointer sdio_set_default_speed

This patch removes function pointer sdio_set_default_speed of wilc_sdio_t
and call linux_sdio_set_default_speed() directly.

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