usb: gadget: net2280: Fix NULL pointer dereference

When DEBUG is enabled driver->driver.name is accessed, but driver
can be NULL

[  174.411689] BUG: unable to handle kernel NULL pointer dereference at 0000000000000040
[  174.429043] RIP: 0010:[<ffffffffa0131ca3>]  [<ffffffffa0131ca3>] net2280_stop+0xa3/0x100 [net2280]
[  174.457910] Call Trace:
[  174.459503]  [<ffffffffa00dd92a>] usb_gadget_remove_driver+0x5a/0xb0 [udc_core]
[  174.462693]  [<ffffffffa00ddd84>] usb_del_gadget_udc+0xb4/0x110 [udc_core]
[  174.464316]  [<ffffffffa012e2bf>] net2280_remove+0x2f/0x1c0 [net2280]

Signed-off-by: Ricardo Ribalda Delgado <ricardo.ribalda@gmail.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>