scsi: ufs: wb: Introduce ufshcd_is_wb_buf_flush_allowed()

The explicit flushing should check the following:

 - UFSHCD_CAP_WB_EN

 - UFSHCI_QUIRK_SKIP_MANUAL_WB_FLUSH_CTRL

Add helper to improve readability.

Link: https://lore.kernel.org/r/20220804075444epcms2p4a0520880262281f02be65ce0fe50602d@epcms2p4


Reviewed-by: Avri Altman <avri.altman@wdc.com>
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Reviewed-by: Stanley Chu <stanley.chu@mediatek.com>
Acked-by: Bean Huo <beanhuo@micron.com>
Signed-off-by: Jinyoung Choi <j-young.choi@samsung.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>