WORKAROUND: usb: gadget: f_acm: Add dummy mode

WORKAROUND: usb: gadget: f_acm: Add dummy mode

With amlogic dwc2, only fixed bytes for fifo can be used because
it is set as 2848 bytes in sram. But Tizen default usb gadget mode
enables mtp, acm, and sdb, and for those interfaces, 3104 bytes
are required. Disabling acm gadget causes usb mode setting fail in
Tizen deviced, so add acm gadget dummy mode which enables acm
gadget in configuration, but not really using any endpoint fifo.

Note: once gadget mode is properly fixed, this change will not be
necessary, so it will be reverted after gadget mode modification
is done in deviced.

Change-Id: I6148a714520642050133b6c32bce666971869826
Signed-off-by: Seung-Woo Kim <sw0312.kim@samsung.com>