b43legacy: Remove unused inline function txring_to_priority()

commit 5d07a3d6 ("b43legacy: Avoid packet losses in the dma worker code")
left behind this.

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20210529115131.6028-1-yuehaibing@huawei.com