usb: dwc3: gadget: change HIRD threshold to 12

First of all, that 28 value makes no sense as
HIRD threshold is a 4-bit value, second of all
it's causing issues for OMAP5.

Using 12 because commit cbc725b3

 (usb: dwc3:
keep default hird threshold value as 4b1100)
had the intention of setting the maximum allowed
value of 0xc.

Also, original code has been wrong forever, so
this should be backported as far back as
possible.

Cc: <stable@vger.kernel.org>
Signed-off-by: Felipe Balbi <balbi@ti.com>