staging: ks7010: refactor ks_wlan_set_power_mgmt function

This commit change logic to simplify conditional paths in
ks_wlan_set_power_mgmt function. It handles invalid's first
and just finally assign a valid value.

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