USB: fix fsl_otg config dependency

USB_GADGET_FSL_USB2 has been changed to USB_FSL_USB2 by commit
193ab2a6 (usb: gadget: allow
multiple gadgets to be built). But old USB_GADGET_FSL_USB2 is
still listed as dependency for fsl_otg driver, so the driver
cannot be selected in the configuration currently. Fix it.

Signed-off-by: Anatolij Gustschin <agust@denx.de>
Signed-off-by: Felipe Balbi <balbi@ti.com>