scsi: ufs: Refactor ufshcd_init/exit_clk_scaling/gating()

ufshcd_hba_exit() is always called after ufshcd_exit_clk_scaling() and
ufshcd_exit_clk_gating(). Move ufshcd_exit_clk_scaling/gating() to
ufshcd_hba_exit(). Meanwhile, add dedicated functions to initialize
and remove sysfs nodes of clock scaling/gating to make the code more
readable. Overall functionality remains same.

Link: https://lore.kernel.org/r/1611137065-14266-3-git-send-email-cang@codeaurora.org
Reviewed-by: Stanley Chu <stanley.chu@mediatek.com>
Signed-off-by: Can Guo <cang@codeaurora.org>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>