scsi: ufs: wb: Add explicit flush sysfs attribute

There is the following quirk to bypass "WB Flush" in Write Booster.

	- UFSHCI_QUIRK_SKIP_MANUAL_WB_FLUSH_CTRL

If this quirk is not set, there is no knob that can control "WB Flush".

There are three flags that control Write Booster Feature:

	1. WB ON/OFF
	2. WB Hibern Flush ON/OFF (implicitly)
	3. WB Flush ON/OFF (explicit)

The sysfs attribute that controls the WB was implemented. (1)

In the case of "Hibern Flush", it is always good to turn on.  Control may
not be required. (2)

Finally, "Flush" may be necessary because the Auto-Hibern8 is not supported
in a specific environment.  So the sysfs attribute that controls this is
necessary. (3)

Link: https://lore.kernel.org/r/20220804075354epcms2p8c21c894b4e28840c5fc651875b7f435f@epcms2p8


Reviewed-by: Avri Altman <avri.altman@wdc.com>
Signed-off-by: Jinyoung Choi <j-young.choi@samsung.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>