usb: gadget: add module.h to u_ether.c

Spotted by Fengguang Wu:
|In file included from drivers/usb/gadget/ether.c:110:0:
|drivers/usb/gadget/u_ether.c:87:21: error: expected ')' before 'uint'
|drivers/usb/gadget/u_ether.c:88:25: error: expected ')' before string constant

This is because u_ether.c relied on having module.h included somewhere.
This isn't the case since composite.c is no longer included.

Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Felipe Balbi <balbi@ti.com>