usb: gadget: f_dfu: Add get_alt function to pass the USB compliance test

Without this function the USB compliance test (USB 2.0 Command Verifier) will
fail in the "Interface Descriptor Test" with this error message:

FAIL
(1.2.51) A successful GetInterface request must return the alternate setting
set by a prior call to SetInterface.

Lets add this function to read back the value so that the DFU device fully
passes the USB compliance test.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Roger Meier <r.meier@siemens.com>
Cc: Samuel Egli <samuel.egli@siemens.com>
Cc: Enrico Leto <enrico.leto@siemens.com>
Acked-by: Heiko Schocher <hs@denx.de>
Cc: Lukasz Majewski <l.majewski@samsung.com>
Cc: Marek Vasut <marex@denx.de>