usb, dfu gadget: switch to dfu mode in dfu_bind

usb, dfu gadget: switch to dfu mode in dfu_bind

- set in to_dfu_mode() f_dfu->dfu_state = DFU_STATE_dfuIDLE
  as after every to_dfu_mode call this is done, so move
  this into to_dfu_mode
- switch in dfu_bind() into dfu mode:
  This fixes wrong "dfu-util -l" output, when calling
  "dfu-util -l" after a board reset, without doing a
  download before. See also discussion here:

http://lists.denx.de/pipermail/u-boot/2013-June/157272.html

Signed-off-by: Heiko Schocher <hs@denx.de>
Cc: Lukasz Majewski <l.majewski@samsung.com>
Cc: Pantelis Antoniou <panto@antoniou-consulting.com>
Cc: Tom Rini <trini@ti.com>
Cc: Kyungmin Park <kyungmin.park@samsung.com>
Cc: Marek Vasut <marex@denx.de>
Cc: Wolfgang Denk <wd@denx.de>
Cc: Samuel Egli <samuel.egli@siemens.com>
Acked-by: Lukasz Majewski <l.majewski@samsung.com>
Tested-by: Lukasz Majewski <l.majewski@samsung.com>