usb: dwc2: gadget: Fix GUSBCFG.USBTRDTIM value

USBTrdTim must be programmed to 0x5 when phy has a UTMI+ 16-bit wide
interface or 0x9 when it has a 8-bit wide interface.
GUSBCFG reset value (Value After Reset: 0x1400) sets USBTrdTim to 0x5.
In case of 8-bit UTMI+, without clearing GUSBCFG.USBTRDTIM mask, USBTrdTim
results in 0xD (0x5 | 0x9).
That's why we need to clear GUSBCFG.USBTRDTIM mask before setting USBTrdTim
value, to ensure USBTrdTim is correctly set in case of 8-bit UTMI+.

Signed-off-by: Amelie Delaunay <amelie.delaunay@st.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>