usb: dwc3: Correct DWC3_DCTL_HIRD_THRES definition

The definition of DWC3_DCTL_HIRD_THRES macro is
completely wrong. It will only work for when we
want to read the register's contents for that bitfield.

Change the macro so that it can be used to writing to
the register, and when we need to read, we can add
extra right shift of 24 bits.

Signed-off-by: Pratyush Anand <pratyush.anand@st.com>

[ balbi@ti.com: add a commit log ]

Signed-off-by: Felipe Balbi <balbi@ti.com>