scsi: ufs: Fix per-LU WB buffer size setting error

Currently we pass sizeof(*d_lun_wbb_au) as param size to request
dLUNumWriteBoosterBufferAllocUnits. In this way we can only get 1 byte
data while dLunNumWriteBoosterBufferAllocUnites size should be 4 bytes.
To fix this issue, we should pass the actual size of that buffer. The
founction ufshcd_get_wb_alloc_units() is redundant, remove it and
directly call ufshcd_read_unit_desc_param() in ufs_get_device_desc()
to read back the dLUNumWriteBoosterBufferAllocUnits.

Change-Id: Ie6eee00dcb1a4c2334abc2a01d3e81994018835a
Signed-off-by: Ziqi Chen <ziqichen@codeaurora.org>