iwmc3200wifi: clarify potentially undefined operation in iwm_scan_ssids

  CC [M]  drivers/net/wireless/iwmc3200wifi/commands.o
drivers/net/wireless/iwmc3200wifi/commands.c: In function ‘iwm_scan_ssids’:
drivers/net/wireless/iwmc3200wifi/commands.c:911:15: warning: operation on ‘iwm->scan_id’ may be undefined

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