USB: imx_udc: Fix IMX UDC gadget general irq handling

Workaround of hw bug in IMX UDC.
This bug causes wrong handling of CFG_CHG interrupt.
Workaround is documented inline source code.

Signed-off-by: Darius Augulis <augulis.darius@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>