staging: wfx: Make some functions static in sta.c

staging: wfx: Make some functions static in sta.c

Fix sparse warnings:

drivers/staging/wfx/sta.c:269:6: warning: symbol 'wfx_update_filtering_work' was not declared. Should it be static?
drivers/staging/wfx/sta.c:475:6: warning: symbol 'wfx_event_handler_work' was not declared. Should it be static?
drivers/staging/wfx/sta.c:521:6: warning: symbol 'wfx_bss_loss_work' was not declared. Should it be static?
drivers/staging/wfx/sta.c:528:6: warning: symbol 'wfx_bss_params_work' was not declared. Should it be static?
drivers/staging/wfx/sta.c:539:6: warning: symbol 'wfx_set_beacon_wakeup_period_work' was not declared. Should it be static?
drivers/staging/wfx/sta.c:732:6: warning: symbol 'wfx_unjoin_work' was not declared. Should it be static?
drivers/staging/wfx/sta.c:794:6: warning: symbol 'wfx_set_cts_work' was not declared. Should it be static?
drivers/staging/wfx/sta.c:1234:6: warning: symbol 'wfx_set_tim_work' was not declared. Should it be static?

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: zhengbin <zhengbin13@huawei.com>
Reviewed-by: Jérôme Pouiller <jerome.pouiller@silabs.com>
Link: https://lore.kernel.org/r/1570629030-29888-4-git-send-email-zhengbin13@huawei.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>