staging: ks7010: refactor ks_wlan_set_sleep_mode function

This commit refactors ks_wlan_set_sleep_mode function
avoiding to use switch-case statement ans using simple
if logic to handle invalid values first. This simplifies
data paths as well as improves readability.

Signed-off-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>