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=4642


Signed-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