staging: wilc1000: remove sdio speed control codes

This patch removes spi speed control related functions and variable. We cannot
get exact clock what we need in this way and it can causes some problem in host
side by setting the clock, so remove the codes.
Speed control codes in spi also will removed in next patch, so it's ok to
remove functions in linux_wlan.c and wilc_wlan.c which also not used anymore.

The Following functions and varialbe are removed.
MAX_SPEED, sdio_default_speed
wilc_bus_set_default_speed
wilc_bus_set_max_speed
linux_sdio_set_speed
linux_sdio_get_speed
wilc_sdio_set_max_speed
wilc_sdio_set_default_speed

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