usb: gadget: u_ether: convert into module

u_ether.c has been #include'd by all gadgets which implement
USB Ethernet functions. In order to add configfs support,
the f_ecm.c, f_eem.c, f_ncm.c, f_subset.c, f_rndis.c need to be
converted into modules and must not be #include'd. Consequently,
the u_ether.c needs to be a module too, in a manner similar
to u_serial.c. The resulting module should not take any parameters,
so they are pushed to the current users of it, that is ether.c,
g_ffs.c, multi.c, ncm.c, nokia.c.

Signed-off-by: Andrzej Pietrasiewicz <andrzej.p@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>