usb: gadget: dfu: discard dead code

Reported by Coverity:
Logically dead code (DEADCODE)
dead_error_line: Execution cannot reach this statement:
(f_dfu->strings + --i).s = ....

If calloc failed, i is still 0 and no need to call free,
so discard the dead code.

Signed-off-by: Peng Fan <van.freenix@gmail.com>
Cc: "Łukasz Majewski" <l.majewski@samsung.com>
Cc: Marek Vasut <marex@denx.de>