tools: env: fw_env: Fix unused-result warning

Fix unused-result warning about fread.

tools/env/fw_env.c: In function ‘find_nvmem_device’:
tools/env/fw_env.c:1751:3: warning: ignoring return value of ‘fread’, declared with attribute warn_unused_result [-Wunused-result]
 1751 |   fread(buf, sizeof(buf), 1, fp);
      |   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Signed-off-by: Jaehoon Chung <jh80.chung@samsung.com>