wifi: brcm80211: Remove unused dma_txflush()

dma_fxflush() has been unused since 2013's
commit 7b2385b9 ("brcmsmac: rework of mac80211 .flush() callback
operation")

Remove it.

Signed-off-by: Dr. David Alan Gilbert <linux@treblig.org>
Acked-by: Arend van Spriel <arend.vanspriel@broadcom.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://patch.msgid.link/20241025204651.244627-1-linux@treblig.org