staging: ks7010: refactor ks_wlan_set_mode function

Most cases which are being handled in the switch-case of
ks_wlan_set_mode function are just returning EINVAL. Avoid
the use of switch-case stament and just use a simple if
to handle those. This decrease LOC as well as improves
readability.

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