Revert "usb: gadget: zero: Add support for interrupt EP"

This reverts commit ef11982d.

That commit creates a problem for some UDCs (at least musb)
where it allocates an endpoints with a 64-byte FIFO, but later
tries to use that same FIFO for 1024-byte packets.

Before implementing this, composite framework needs to be
modified so we only allocate endpoints after we know negotiated
speed, however that needs quite a bit of extra work.

Cc: <stable@vger.kernel.org> # v3.18+
Signed-off-by: Felipe Balbi <balbi@ti.com>