flash_image.c: fix `clang` build

[trofi]

Without the change the build fails on `clang as:

/build/bladeRF/host/utilities/bladeRF-cli/src/cmd/flash_image.c:71:35:
  error: overlapping comparisons always evaluate to true [-Werror,-Wtautological-overlap-compare]
   71 |                 if (val[i] >= 'a' || val[i] <= 'f') {
      |                     ~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~

I think it flags real error. Let's use && to catch the range.

[CLAassistant]

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