USB: DWC3: Issue device soft reset before core soft reset

Synopsys specification clearly states under section "Device Power-On or
Soft Reset" that DCTL.CSftRst=1 should be first step.  So, just follow
what specification says.

Signed-off-by: Pratyush Anand <pratyush.anand@st.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>