ipw2100: mark ipw2100_pm_qos_req static

  CHECK   drivers/net/wireless/ipw2x00/ipw2100.c
drivers/net/wireless/ipw2x00/ipw2100.c:177:28: warning: symbol 'ipw2100_pm_qos_req' was not declared. Should it be static?

Signed-off-by: John W. Linville <linville@tuxdriver.com>