usb: gadget: use consistent naming scheme for usb function modules

In order to convert to configfs the USB functions need to be converted
to the new interface from Sebastian, which also requires turning them
into separate modules. Some of these modules will consist of just
one object file, e.g. f_ncm.o. But some of the modules will eventually
consist of more than one object file, e.g. for mass storage there
will be f_mass_storage.o and storage_common.o. The resulting module
cannot be called f_mass_storage.ko due to cyclic dependency. This patch
introduces a naming scheme for the said resulting modules:

usb_f_xxxxxx.ko

e.g. usb_f_mass_storage.ko, usb_f_ss_lb.ko, usb_f_rndis.ko etc.

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>