usb: gadget: net2272: finally convert "CONFIG_USB_GADGET_NET2272_DMA"

The Kconfig symbol USB_GADGET_NET2272_DMA was renamed to USB_NET2272_DMA
in commit 193ab2a6070039e7ee2b9b9bebea754a7c52fd1b ("usb: gadget: allow
multiple gadgets to be built"). That commit did not convert the only
occurrence of the corresponding Kconfig macro. Convert that macro now.

Signed-off-by: Paul Bolle <pebolle@tiscali.nl>
Signed-off-by: Felipe Balbi <balbi@ti.com>