wifi: b43legacy: Remove the unused function prev_slot()

The function prev_slot is defined in the dma.c file, but not called
elsewhere, so remove this unused function.

drivers/net/wireless/broadcom/b43legacy/dma.c:130:19: warning: unused function 'prev_slot'.
Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Link: https://bugzilla.openanolis.cn/show_bug.cgi?id=4642Signed-off-by: Jiapeng Chong <jiapeng.chong@linux.alibaba.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Acked-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20230330021841.67724-1-jiapeng.chong@linux.alibaba.com

drivers/net/wireless/broadcom/b43legacy/dma.c

@@ -127,14 +127,6 @@ static inline int next_slot(struct b43legacy_dmaring *ring, int slot)
 	return slot + 1;
 }
 
-static inline int prev_slot(struct b43legacy_dmaring *ring, int slot)
-{
-	B43legacy_WARN_ON(!(slot >= 0 && slot <= ring->nr_slots - 1));
-	if (slot == 0)
-		return ring->nr_slots - 1;
-	return slot - 1;
-}
-
 #ifdef CONFIG_B43LEGACY_DEBUG
 static void update_max_used_slots(struct b43legacy_dmaring *ring,
 				  int current_used_slots)