usb:gadget:f_thor: code cleanup in function download_tail()

In thor's download_tail() function, dfu_get_entity() is called
before each dfu_write() call and the returned entity pointers
are the same. So dfu_get_entity() can be called just once and
this patch changes this.

Signed-off-by: Przemyslaw Marczak <p.marczak@samsung.com>
Cc: Lukasz Majewski <l.majewski@samsung.com>
Cc: Marek Vasut <marex@denx.de>
Cc: Heiko Schocher <hs@denx.de>
Cc: Tom Rini <trini@ti.com>