image-fit: fix fit_image_load() OS check

image-fit: fix fit_image_load() OS check

Commit 62afc601883e788f3f22291202d5b2a23c1a8b06 introduced fpga image load via
bootm but broke the OS check in fit_image_load().

This commit removes following compiler warning:

---8<---
In file included from tools/common/image-fit.c:1:
/Volumes/devel/u-boot/tools/../common/image-fit.c:1715:39: warning: use of logical '||' with constant operand [-Wconstant-logical-operand]
        os_ok = image_type == IH_TYPE_FLATDT || IH_TYPE_FPGA ||
                                             ^  ~~~~~~~~~~~~
/Volumes/devel/u-boot/tools/../common/image-fit.c:1715:39: note: use '|' for a bitwise operation
        os_ok = image_type == IH_TYPE_FLATDT || IH_TYPE_FPGA ||
                                             ^~
                                             |
1 warning generated.
--->8---

Signed-off-by: Andreas Bießmann <andreas@biessmann.org>
Cc: Michal Simek <michal.simek@xilinx.com>
Acked-by: Michal Simek <michal.simek@xilinx.com>