iwlwifi: make scan workqueue functions static

We do not need export iwl_bg.*scan.*() functions just for initialize
workqueue in other module. Making that functions static helps with
iwl-scan.c code review a bit.

Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>