qemu: dfu: Correct memset call in set_dfu_alt_info

qemu: dfu: Correct memset call in set_dfu_alt_info

When building with clang, we see:
board/emulation/common/qemu_dfu.c:51:24: warning: 'memset' call operates on objects of type 'char' while the size is based on a different type 'char *' [-Wsizeof-pointer-memaccess]

As we're calling memset with the length set to the size of the pointer
and not the size of the buffer. Correct this with a call of the size of
the buffer itself.

Signed-off-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Acked-by: Sughosh Ganu <sughosh.ganu@linaro.org>