ufs: fix reference counting of W-LUs

UFS driver adds three well known LUs in the initialization, but those
reference counts are not decremented, so it makes ufshcd module
impossible to unload.

This fixes it by putting scsi_device_put() in the initalization, and in
order to protect concurrent access to hba->sdev_ufs_device (UFS Device
W-LU) from manual delete, increment the reference count while requesting
device power mode setting.

The rest of W-LUs (hba->sdev_boot and hba->sdev_rpmb) are not directly
used from driver, so these references in struct ufs_hba are removed.

Signed-off-by: Akinobu Mita <mita@fixstars.com>
Reviewed-by: Maya Erez <merez@codeaurora.org>
Reviewed-by: Dolev Raviv <draviv@codeaurora.org>
Signed-off-by: Christoph Hellwig <hch@lst.de>