usb: gadget: remove useless parameter in alloc_ep_req()

The default_length parameter of alloc_ep_req was not really necessary
and gadget drivers would almost always create an inline function to pass
the same value to len and default_len.

This patch removes that parameter and updates all calls to alloc_ep_req() to
use the new API.

Signed-off-by: Felipe F. Tonello <eu@felipetonello.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>