usb: gadget: net2272: fix sparse warnings

Fix the following sparse warnings:

drivers/usb/gadget/net2272.c:916:13: warning: Using plain integer as NULL pointer
drivers/usb/gadget/net2272.c:1624:9: warning: Using plain integer as NULL pointer
drivers/usb/gadget/net2272.c:1552:30: warning: right shift by bigger than source value

Note that the last warning is an actual bug,
since ep->dma is a one bit value which is
shifted by one bit in code.

Signed-off-by: Felipe Balbi <balbi@ti.com>