usb: dfu: add static alt num count in dfu_config_entities()

Thanks to this multiple call of function dfu_config_entities()
gives continuous dfu alt numbering until call dfu_free_entities().

This allows to store dfu entities in multiple variables.

Signed-off-by: Przemyslaw Marczak <p.marczak@samsung.com>
Acked-by: Łukasz Majewski <l.majewski@samsung.com>
Tested-by: Heiko Schocher <hs@denx.de>
Signed-off-by: Minkyu Kang <mk7.kang@samsung.com>