ufs: Use 'TASK_TAG' to construct the ucd_req_ptr->header.dword_0

Instead of using the hard-coded value of 0x1f, use 'TASK_TAG'
macro instead to construct the ucd_req_ptr->header.dword_0

This is in sync with what the Linux UFS driver does, i.e.
set the byte0 equal to TASK_TAG (see [1]).

Setting it to a fixed value of 0x1f is wrong as we define
TASK_TAG as 0 inside u-boot ufs framework. So, instead we
should  use the macro value directly.

[1]. https://github.com/torvalds/linux/blob/master/drivers/ufs/core/ufshcd.c#L2705



Signed-off-by: Bhupesh Sharma <bhupesh.sharma@linaro.org>