libertas: fix non static symbol warning

Fixes the following sparse warning:

drivers/net/wireless/marvell/libertas/cfg.c:2047:5: warning:
 symbol 'lbs_set_power_mgmt' was not declared. Should it be static?

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>