staging: wfx: Make function 'wfx_tx_queue_get' static

Fix sparse warnings:

drivers/staging/wfx/queue.c:218:16: warning: symbol 'wfx_tx_queue_get' 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/1570629030-29888-3-git-send-email-zhengbin13@huawei.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>