scsi: ufs: Cleanup WriteBooster feature

Small cleanup as below items,

 1. Use ufshcd_is_wb_allowed() directly instead of ufshcd_wb_sup() since
    ufshcd_wb_sup() just returns the result of ufshcd_is_wb_allowed().

 2. In ufshcd_suspend(), "else if (!ufshcd_is_runtime_pm(pm_op)) can be
    simplified to "else" since both have the same meaning.

This patch does not change any functionality.

Link: https://lore.kernel.org/r/20200508080115.24233-9-stanley.chu@mediatek.com
Reviewed-by: Avri Altman <avri.altman@wdc.com>
Reviewed-by: Asutosh Das <asutoshd@codeaurora.org>
Signed-off-by: Stanley Chu <stanley.chu@mediatek.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>