usb: gadget: lpc32xx_udc: Fix compatibility with STOTG04

The STOTG04 is an replacement for ISP1301.

Most of the registers on STOTG04 are the same as on ISP1301, but the
register ISP1301_I2C_OTG_CONTROL_2 (address 0x10) doesn't exist on the
ST part.

This is a work around for this by using the interrupt source register that
should behave the same on both parts and has the needed information.

Tested-by: Roland Stigge <stigge@antcom.de>
Signed-off-by: Alexandre Pereira da Silva <aletes.xgr@gmail.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>