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

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 <[email protected]>
Cc: Roger Meier <[email protected]>
Cc: Samuel Egli <[email protected]>
Cc: Enrico Leto <[email protected]>
Acked-by: Heiko Schocher <[email protected]>
Cc: Lukasz Majewski <[email protected]>
Cc: Marek Vasut <[email protected]>