scsi: ufs: Cleanup WB buffer flush toggle implementation

Delete ufshcd_wb_buf_flush_enable() and ufshcd_wb_buf_flush_disable(). Move
the implementation into ufshcd_wb_toggle_flush().

Link: https://lore.kernel.org/r/20210121185736.12471-1-huobean@gmail.com
Reviewed-by: Stanley Chu <stanley.chu@mediatek.com>
Reviewed-by: Can Guo <cang@codeaurora.org>
Signed-off-by: Bean Huo <beanhuo@micron.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>